Mention exact previous branch name in rollback checkout dialog

Instead of less clear "previous branch".

Mentioned in IDEA-163836.

plugins/git4idea/src/git4idea/branch/GitCheckoutOperation.java

@@ -26,6 +26,7 @@ import com.intellij.openapi.vcs.VcsNotifier;
 import com.intellij.openapi.vcs.changes.Change;
 import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.util.ArrayUtil;
+import com.intellij.util.containers.ContainerUtil;
 import git4idea.GitUtil;
 import git4idea.commands.*;
 import git4idea.config.GitVcsSettings;
@@ -38,6 +39,7 @@ import javax.swing.event.HyperlinkEvent;
 import java.util.Collection;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.stream.Collectors;
 
 import static git4idea.util.GitUIUtil.code;
 import static java.util.Arrays.stream;
@@ -53,8 +55,6 @@ import static java.util.Arrays.stream;
  */
 class GitCheckoutOperation extends GitBranchOperation {
 
-  public static final String ROLLBACK_PROPOSAL_FORMAT = "You may rollback (checkout back to previous branch) not to let branches diverge.";
-
   @NotNull private final String myStartPointReference;
   private final boolean myDetach;
   private final boolean myRefShouldBeValid;
@@ -192,8 +192,15 @@ class GitCheckoutOperation extends GitBranchOperation {
   @NotNull
   @Override
   protected String getRollbackProposal() {
+    Collection<String> distinctPrevBranches = getSuccessfulRepositories().stream().
+      map(myCurrentHeads::get).
+      distinct().
+      collect(Collectors.toList());
+    String previousBranch = distinctPrevBranches.size() == 1 ? ContainerUtil.getFirstItem(distinctPrevBranches) : "previous branch";
+    String rollBackProposal = "You may rollback (checkout back to " + previousBranch + ") not to let branches diverge.";
     return "However checkout has succeeded for the following " + repositories() + ":<br/>" +
-           successfulRepositoriesJoined() + "<br/>" + ROLLBACK_PROPOSAL_FORMAT;
+           successfulRepositoriesJoined() + "<br/>" +
+           rollBackProposal;
   }
 
   @NotNull