[git] IDEA-87609 When updating via Rebase, use merge instead of rebase, if it is fast-forward.

Before starting the normal update procedure (with possible stash-unstash), try to perform a fast-forward merge instead of rebase, because it is equal to fast-forward rebase, but is faster, because doesn't require stash-unstash (unless the uncommitted files would be overwritten by merge)
Any failure in 'git merge --ff-only' means, than this repository will then be updated in a usual manner.
No pre-checking if a fast-forward merge is possible is done, because it is fast enough to execute the command and get the error.

plugins/git4idea/src/git4idea/branch/GitMergeOperation.java

@@ -93,7 +93,7 @@ class GitMergeOperation extends GitBranchOperation {
       GitSimpleEventDetector mergeConflict = new GitSimpleEventDetector(GitSimpleEventDetector.Event.MERGE_CONFLICT);
       GitSimpleEventDetector alreadyUpToDateDetector = new GitSimpleEventDetector(GitSimpleEventDetector.Event.ALREADY_UP_TO_DATE);
 
-      GitCommandResult result = myGit.merge(repository, myBranchToMerge,
+      GitCommandResult result = myGit.merge(repository, myBranchToMerge, Collections.<String>emptyList(),
                                           localChangesOverwrittenByMerge, unmergedFiles, untrackedOverwrittenByMerge, mergeConflict,
                                           alreadyUpToDateDetector);
       if (result.success()) {
@@ -238,7 +238,7 @@ class GitMergeOperation extends GitBranchOperation {
   private boolean doMerge(@NotNull Collection<GitRepository> repositories) {
     for (GitRepository repository : repositories) {
       GitSimpleEventDetector mergeConflict = new GitSimpleEventDetector(GitSimpleEventDetector.Event.MERGE_CONFLICT);
-      GitCommandResult result = myGit.merge(repository, myBranchToMerge, mergeConflict);
+      GitCommandResult result = myGit.merge(repository, myBranchToMerge, Collections.<String>emptyList(), mergeConflict);
       if (!result.success()) {
         if (mergeConflict.hasHappened()) {
           myConflictedRepositories.put(repository, Boolean.TRUE);

plugins/git4idea/src/git4idea/commands/Git.java

@@ -49,7 +49,8 @@ public interface Git {
   GitCommandResult clone(@NotNull Project project, @NotNull File parentDirectory, @NotNull String url, @NotNull String clonedDirectoryName);
 
   @NotNull
-  GitCommandResult merge(@NotNull GitRepository repository, @NotNull String branchToMerge, @NotNull GitLineHandlerListener... listeners);
+  GitCommandResult merge(@NotNull GitRepository repository, @NotNull String branchToMerge, @Nullable List<String> additionalParams,
+                         @NotNull GitLineHandlerListener... listeners);
 
   @NotNull
   GitCommandResult checkout(@NotNull GitRepository repository, @NotNull String reference, @Nullable String newBranch, boolean force,

plugins/git4idea/src/git4idea/commands/GitImpl.java

@@ -139,10 +139,13 @@ public class GitImpl implements Git {
   @Override
   @NotNull
   public GitCommandResult merge(@NotNull GitRepository repository, @NotNull String branchToMerge,
-                                       @NotNull GitLineHandlerListener... listeners) {
+                                @Nullable List<String> additionalParams, @NotNull GitLineHandlerListener... listeners) {
     final GitLineHandler mergeHandler = new GitLineHandler(repository.getProject(), repository.getRoot(), GitCommand.MERGE);
     mergeHandler.setSilent(false);
     mergeHandler.addParameters(branchToMerge);
+    if (additionalParams != null) {
+      mergeHandler.addParameters(additionalParams);
+    }
     for (GitLineHandlerListener listener : listeners) {
       mergeHandler.addLineListener(listener);
     }

plugins/git4idea/src/git4idea/update/GitRebaseUpdater.java

@@ -24,12 +24,15 @@ import com.intellij.openapi.vcs.VcsException;
 import com.intellij.openapi.vcs.update.UpdatedFiles;
 import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.util.ui.UIUtil;
+import git4idea.GitBranch;
+import git4idea.GitUtil;
 import git4idea.PlatformFacade;
 import git4idea.branch.GitBranchPair;
 import git4idea.commands.*;
 import git4idea.merge.GitConflictResolver;
 import git4idea.rebase.GitRebaseProblemDetector;
 import git4idea.rebase.GitRebaser;
+import git4idea.repo.GitRepository;
 import git4idea.util.GitUIUtil;
 import git4idea.util.UntrackedFilesNotifier;
 import org.jetbrains.annotations.NotNull;
@@ -60,8 +63,7 @@ public class GitRebaseUpdater extends GitUpdater {
 
   protected GitUpdateResult doUpdate() {
     LOG.info("doUpdate ");
-    GitBranchPair gitBranchPair = myTrackedBranches.get(myRoot);
-    String remoteBranch = gitBranchPair.getDest().getName();
+    String remoteBranch = getRemoteBranchToMerge();
 
     final GitLineHandler rebaseHandler = new GitLineHandler(myProject, myRoot, GitCommand.REBASE);
     rebaseHandler.addParameters(remoteBranch);
@@ -99,6 +101,15 @@ public class GitRebaseUpdater extends GitUpdater {
     return updateResult.get();
   }
 
+  @NotNull
+  private String getRemoteBranchToMerge() {
+    GitBranchPair gitBranchPair = myTrackedBranches.get(myRoot);
+    GitBranch dest = gitBranchPair.getDest();
+    LOG.assertTrue(dest != null, String.format("Destination branch is null for source branch %s in %s",
+                                               gitBranchPair.getBranch().getName(), myRoot));
+    return dest.getName();
+  }
+
   private GitUpdateResult handleRebaseFailure(GitLineHandler pullHandler,
                                               GitRebaseProblemDetector rebaseConflictDetector,
                                               final GitMessageWithFilesDetector untrackedWouldBeOverwrittenDetector) {
@@ -159,6 +170,17 @@ public class GitRebaseUpdater extends GitUpdater {
     return "Rebase updater";
   }
 
+  public boolean fastForwardMerge() {
+    LOG.info("Trying fast-forward merge for " + myRoot);
+    GitRepository repository = GitUtil.getRepositoryManager(myProject).getRepositoryForRoot(myRoot);
+    if (repository == null) {
+      LOG.error("Repository is null for " + myRoot);
+      return false;
+    }
+    GitCommandResult result = myGit.merge(repository, getRemoteBranchToMerge(), Collections.singletonList("--ff-only"));
+    return result.success();
+  }
+
   private static class MyConflictResolver extends GitConflictResolver {
     private final GitRebaser myRebaser;
     private final VirtualFile myRoot;

plugins/git4idea/src/git4idea/update/GitUpdateProcess.java

@@ -134,7 +134,7 @@ public class GitUpdateProcess {
 
   @NotNull
   private GitUpdateResult updateImpl(@NotNull UpdateMethod updateMethod, ContinuationContext context) {
-    final Map<VirtualFile, GitUpdater> updaters;
+    Map<VirtualFile, GitUpdater> updaters;
     try {
       updaters = defineUpdaters(updateMethod);
     }
@@ -144,6 +144,8 @@ public class GitUpdateProcess {
       return GitUpdateResult.ERROR;
     }
 
+    updaters = tryFastForwardMergeForRebaseUpdaters(updaters);
+
     if (updaters.isEmpty()) {
       return GitUpdateResult.NOTHING_TO_UPDATE;
     }
@@ -204,6 +206,22 @@ public class GitUpdateProcess {
     return compoundResult;
   }
 
+  @NotNull
+  private static Map<VirtualFile, GitUpdater> tryFastForwardMergeForRebaseUpdaters(@NotNull Map<VirtualFile, GitUpdater> updaters) {
+    Map<VirtualFile, GitUpdater> modifiedUpdaters = new HashMap<VirtualFile, GitUpdater>();
+    for (Map.Entry<VirtualFile, GitUpdater> updaterEntry : updaters.entrySet()) {
+      GitUpdater updater = updaterEntry.getValue();
+      if (updater instanceof GitRebaseUpdater) {
+        GitRebaseUpdater rebaseUpdater = (GitRebaseUpdater) updater;
+        if (rebaseUpdater.fastForwardMerge()) {
+          continue;
+        }
+      }
+      modifiedUpdaters.put(updaterEntry.getKey(), updaterEntry.getValue());
+    }
+    return modifiedUpdaters;
+  }
+
   @NotNull
   private Map<VirtualFile, GitUpdater> defineUpdaters(@NotNull UpdateMethod updateMethod) throws VcsException {
     final Map<VirtualFile, GitUpdater> updaters = new HashMap<VirtualFile, GitUpdater>();

plugins/git4idea/tests/git4idea/test/MockGit.groovy

@@ -88,7 +88,8 @@ class MockGit implements Git {
 
   @NotNull
   @Override
-  GitCommandResult merge(@NotNull GitRepository repository, @NotNull String branchToMerge, @NotNull GitLineHandlerListener... listeners) {
+  GitCommandResult merge(@NotNull GitRepository repository, @NotNull String branchToMerge, @Nullable List<String> additionalParams,
+                         @NotNull GitLineHandlerListener... listeners) {
     throw new UnsupportedOperationException()
   }