Defer object flushing when merging

Mergers in the most recent JGit know how to read previously-inserted
objects back, so a series of merges does not require aggressively
flushing the objects. Instead, flush objects in MergeOp just before
updating the relevant refs.

Change-Id: Ia85c5114e86739b4a480bea65f3413fe47cf572d

gerrit-server/src/main/java/com/google/gerrit/server/git/MergeOp.java

@@ -430,13 +430,11 @@ public class MergeOp {
       String m = "Error opening repository \"" + name.get() + '"';
       throw new MergeException(m, err);
     }
-
-    rw = CodeReviewCommit.newRevWalk(repo);
+    inserter = repo.newObjectInserter();
+    rw = CodeReviewCommit.newRevWalk(inserter.newReader());
     rw.sort(RevSort.TOPO);
     rw.sort(RevSort.COMMIT_TIME_DESC, true);
     canMergeFlag = rw.newFlag("CAN_MERGE");
-
-    inserter = repo.newObjectInserter();
   }
 
   private RefUpdate openBranch()
@@ -674,6 +672,11 @@ public class MergeOp {
             + destProject.getProject().getName(), e);
       }
     }
+    try {
+      inserter.flush();
+    } catch (IOException e) {
+      throw new MergeException("Cannot flush merge results", e);
+    }
 
     branchUpdate.setRefLogIdent(refLogIdent);
     branchUpdate.setForceUpdate(false);