Use changeRefsById to track existing revisions

Most processing paths build up the same sets of ObjectIds, by scanning
through all refs/changes/ and copying them into a temporary set.

Reuse an existing map that is cached after first use and also supports
the autoCloseChanges code path.

It is unlikely that both refs/for/* and refs/heads/* are pushed at the
same time and need to use the map, so there may not be very much reuse
of the data structure within a single execution of ReceiveCommits.

This commit mostly shrinks the code a bit smaller by removing a very
simple redundant processing path.

Change-Id: If2ffd544c821d184593d729fe4ebef77de1860ee

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

@@ -1467,7 +1467,6 @@ public class ReceiveCommits {
     walk.sort(RevSort.TOPO);
     walk.sort(RevSort.REVERSE, true);
     try {
-      Set<ObjectId> existing = Sets.newHashSet();
       walk.markStart(walk.parseCommit(magicBranch.cmd.getNewId()));
       if (magicBranch.baseCommit != null) {
         for (RevCommit c : magicBranch.baseCommit) {
@@ -1480,10 +1479,10 @@ public class ReceiveCommits {
       } else {
         markHeadsAsUninteresting(
             walk,
-            existing,
             magicBranch.ctl != null ? magicBranch.ctl.getRefName() : null);
       }
 
+      Set<ObjectId> existing = changeRefsById().keySet();
       List<ChangeLookup> pending = Lists.newArrayList();
       final Set<Change.Key> newChangeIds = new HashSet<>();
       final int maxBatchChanges =
@@ -1602,23 +1601,15 @@ public class ReceiveCommits {
     }
   }
 
-  private void markHeadsAsUninteresting(
+  private void markHeadsAsUninteresting(RevWalk rw, @Nullable String forRef) {
-      final RevWalk walk,
-      Set<ObjectId> existing,
-      @Nullable String forRef) {
     for (Ref ref : allRefs.values()) {
-      if (ref.getObjectId() == null) {
+      if ((ref.getName().startsWith(R_HEADS) || ref.getName().equals(forRef))
-        continue;
+          && ref.getObjectId() != null) {
-      } else if (ref.getName().startsWith(REFS_CHANGES)) {
-        existing.add(ref.getObjectId());
-      } else if (ref.getName().startsWith(R_HEADS)
-          || (forRef != null && forRef.equals(ref.getName()))) {
         try {
-          walk.markUninteresting(walk.parseCommit(ref.getObjectId()));
+          rw.markUninteresting(rw.parseCommit(ref.getObjectId()));
         } catch (IOException e) {
           log.warn(String.format("Invalid ref %s in %s",
               ref.getName(), project.getName()), e);
-          continue;
         }
       }
     }
@@ -2287,12 +2278,11 @@ public class ReceiveCommits {
     walk.reset();
     walk.sort(RevSort.NONE);
     try {
-      Set<ObjectId> existing = Sets.newHashSet();
       walk.markStart(walk.parseCommit(cmd.getNewId()));
-      markHeadsAsUninteresting(walk, existing, cmd.getRefName());
+      markHeadsAsUninteresting(walk, cmd.getRefName());
 
-      RevCommit c;
+      Set<ObjectId> existing = changeRefsById().keySet();
-      while ((c = walk.next()) != null) {
+      for (RevCommit c; (c = walk.next()) != null;) {
         if (existing.contains(c)) {
           continue;
         } else if (!validCommit(ctl, cmd, c)) {