Set patchset description to 'Rebase' when rebasing

Change-Id: I9e27b211776fd4716a7e9af9dc201fc6fa1defe5

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java

@@ -391,7 +391,7 @@ public class ChangeIT extends AbstractDaemonTest {
     ChangeInfo c2 = gApi.changes().id(changeId).get();
     assertThat(c2.revisions.get(c2.currentRevision)._number).isEqualTo(2);
 
-    // ...and the committer should be correct
+    // ...and the committer and description should be correct
     ChangeInfo info = gApi.changes()
         .id(changeId).get(EnumSet.of(
             ListChangesOption.CURRENT_REVISION,
@@ -400,6 +400,9 @@ public class ChangeIT extends AbstractDaemonTest {
         info.currentRevision).commit.committer;
     assertThat(committer.name).isEqualTo(admin.fullName);
     assertThat(committer.email).isEqualTo(admin.email);
+    String description = info.revisions.get(
+        info.currentRevision).description;
+    assertThat(description).isEqualTo("Rebase");
 
     // Rebasing the second change again should fail
     exception.expect(ResourceConflictException.class);

gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java

@@ -92,6 +92,7 @@ public class PatchSetInserter extends BatchUpdate.Op {
 
   // Fields exposed as setters.
   private String message;
+  private String description;
   private CommitValidators.Policy validatePolicy =
       CommitValidators.Policy.GERRIT;
   private boolean draft;
@@ -145,6 +146,11 @@ public class PatchSetInserter extends BatchUpdate.Op {
     return this;
   }
 
+  public PatchSetInserter setDescription(String description) {
+    this.description = description;
+    return this;
+  }
+
   public PatchSetInserter setValidatePolicy(CommitValidators.Policy validate) {
     this.validatePolicy = checkNotNull(validate);
     return this;
@@ -229,7 +235,7 @@ public class PatchSetInserter extends BatchUpdate.Op {
       }
     }
     patchSet = psUtil.insert(db, ctx.getRevWalk(), ctx.getUpdate(psId),
-        psId, commit, draft, newGroups, null, null);
+        psId, commit, draft, newGroups, null, description);
 
     if (notify != NotifyHandling.NONE) {
       oldReviewers = approvalsUtil.getReviewers(db, ctl.getNotes());

gerrit-server/src/main/java/com/google/gerrit/server/change/RebaseChangeOp.java

@@ -173,6 +173,7 @@ public class RebaseChangeOp extends BatchUpdate.Op {
         ctx.getRepository(), ctl.getChange().currentPatchSetId());
     patchSetInserter = patchSetInserterFactory
         .create(ctl, rebasedPatchSetId, rebasedCommit)
+        .setDescription("Rebase")
         .setDraft(originalPatchSet.isDraft())
         .setNotify(NotifyHandling.NONE)
         .setFireRevisionCreated(fireRevisionCreated)