[log] Allow null roots in the GraphTableModel.

Some rows may be empty (contain no commits): for instance,
in the case of graph filtered by branch, when there will be just
graph fragments drawn.

Therefore it is not an error to have a null node,
and therefore it is not an error to receive the FAKE_ROOT in the
color manager.

Supply correct root to the LoadingDetails (the root is known because
is taken from the Branch of a Node).

platform/vcs-log/impl/src/com/intellij/vcs/log/data/DataGetter.java

@@ -80,7 +80,7 @@ public abstract class DataGetter<T extends VcsShortCommitDetails> implements Dis
   @NotNull
   private T loadingDetails(Node node, Hash hash) {
     TaskDescriptor descriptor = runLoadAroundCommitData(node);
-    T loadingDetails = (T)new LoadingDetails(hash, descriptor.getTaskNum());
+    T loadingDetails = (T)new LoadingDetails(hash, descriptor.getTaskNum(), node.getBranch().getRepositoryRoot());
     return loadingDetails;
   }
 
@@ -145,7 +145,7 @@ public abstract class DataGetter<T extends VcsShortCommitDetails> implements Dis
         // fill the cache with temporary "Loading" values to avoid producing queries for each commit that has not been cached yet,
         // even if it will be loaded within a previous query
         if (!myCache.isKeyCached(hash)) {
-          myCache.put(hash, (T)new LoadingDetails(hash, taskNumber));
+          myCache.put(hash, (T)new LoadingDetails(hash, taskNumber, commitNode.getBranch().getRepositoryRoot()));
         }
       }
     }

platform/vcs-log/impl/src/com/intellij/vcs/log/data/LoadingDetails.java

@@ -1,9 +1,9 @@
 package com.intellij.vcs.log.data;
 
 import com.intellij.openapi.vcs.changes.Change;
+import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.vcs.log.Hash;
 import com.intellij.vcs.log.impl.VcsFullCommitDetailsImpl;
-import com.intellij.vcs.log.ui.tables.AbstractVcsLogTableModel;
 import org.jetbrains.annotations.NotNull;
 
 import java.util.Collections;
@@ -18,9 +18,8 @@ public class LoadingDetails extends VcsFullCommitDetailsImpl {
 
   private final long myLoadingTaskIndex;
 
-  public LoadingDetails(@NotNull Hash hash, long loadingTaskIndex) {
-    super(hash, Collections.<Hash>emptyList(), -1, AbstractVcsLogTableModel.UNKNOWN_ROOT,
-          "Loading...", "", "", "", "", "", -1, Collections.<Change>emptyList());
+  public LoadingDetails(@NotNull Hash hash, long loadingTaskIndex, @NotNull VirtualFile root) {
+    super(hash, Collections.<Hash>emptyList(), -1, root, "Loading...", "", "", "", "", "", -1, Collections.<Change>emptyList());
     myLoadingTaskIndex = loadingTaskIndex;
   }
 

platform/vcs-log/impl/src/com/intellij/vcs/log/ui/VcsLogColorManagerImpl.java

@@ -5,6 +5,7 @@ import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.ui.JBColor;
 import com.intellij.util.containers.ContainerUtil;
 import com.intellij.util.ui.UIUtil;
+import com.intellij.vcs.log.ui.tables.AbstractVcsLogTableModel;
 import org.jetbrains.annotations.NotNull;
 
 import java.awt.*;
@@ -51,6 +52,9 @@ public class VcsLogColorManagerImpl implements VcsLogColorManager {
   @NotNull
   @Override
   public Color getRootColor(@NotNull VirtualFile root) {
+    if (root == AbstractVcsLogTableModel.FAKE_ROOT) {
+      return UIUtil.getTableBackground();
+    }
     Color color = myRoots2Colors.get(root);
     if (color == null) {
       LOG.error("No color record for root " + root + ". All roots: " + myRoots2Colors);

platform/vcs-log/impl/src/com/intellij/vcs/log/ui/tables/AbstractVcsLogTableModel.java

@@ -17,7 +17,7 @@ import java.util.List;
  */
 public abstract class AbstractVcsLogTableModel<T> extends AbstractTableModel {
 
-  public static final VirtualFile UNKNOWN_ROOT = NullVirtualFile.INSTANCE;
+  public static final VirtualFile FAKE_ROOT = NullVirtualFile.INSTANCE;
 
   public static final int ROOT_COLUMN = 0;
   public static final int COMMIT_COLUMN = 1;

platform/vcs-log/impl/src/com/intellij/vcs/log/ui/tables/GraphTableModel.java

@@ -100,13 +100,7 @@ public class GraphTableModel extends AbstractVcsLogTableModel<GraphCommitCell> {
   @Override
   protected VirtualFile getRoot(int rowIndex) {
     Node commitNode = myDataPack.getGraphModel().getGraph().getCommitNodeInRow(rowIndex);
-    if (commitNode != null) {
-      return commitNode.getBranch().getRepositoryRoot();
-    }
-    else {
-      LOG.error("Couldn't identify commit node at " + rowIndex);
-      return UNKNOWN_ROOT;
-    }
+    return commitNode != null ? commitNode.getBranch().getRepositoryRoot() : FAKE_ROOT;
   }
 
   @NotNull

platform/vcs-log/impl/src/com/intellij/vcs/log/ui/tables/NoGraphTableModel.java

@@ -84,7 +84,7 @@ public class NoGraphTableModel extends AbstractVcsLogTableModel<CommitCell> {
     }
     else {
       LOG.error("Couldn't identify root for commit at " + rowIndex, new Attachment("loaded_commits", myCommits.toString()));
-      return UNKNOWN_ROOT;
+      return FAKE_ROOT;
     }
   }