Cache sorted ref lists in CommitData.getRefsById

getRefsById re-sorted the refs pointing at a commit on every call,
even though the per-prefix index it looks them up in is built once
and never changes afterward. Sort each id's refs when the index is
built instead, so repeated lookups for the same prefix reuse the
already-sorted list rather than re-sorting on every commit rendered.

Change-Id: I8027c900e97f471dd6ba91b6b1b2ceeaca0afb2b

java/com/google/gitiles/CommitData.java

diff --git a/java/com/google/gitiles/CommitData.java b/java/com/google/gitiles/CommitData.java
index 25797d2..086e580 100644
--- a/java/com/google/gitiles/CommitData.java
+++ b/java/com/google/gitiles/CommitData.java
@@ -87,7 +87,7 @@
 
   static class Builder {
     private ArchiveFormat archiveFormat;
-    private Map<String, Map<AnyObjectId, Set<Ref>>> refsByIdByPrefix;
+    private Map<String, Map<AnyObjectId, List<Ref>>> refsByIdByPrefix;
     private static final int MAX_NOTE_SIZE = 524288;
 
     Builder setArchiveFormat(@Nullable ArchiveFormat archiveFormat) {
@@ -210,11 +210,11 @@
       if (refsByIdByPrefix == null) {
         refsByIdByPrefix = new HashMap<>();
       }
-      Map<AnyObjectId, Set<Ref>> refsById = refsByIdByPrefix.get(prefix);
+      Map<AnyObjectId, List<Ref>> refsById = refsByIdByPrefix.get(prefix);
       if (refsById == null) {
         // Index only refs under this namespace. getAllRefsByPeeledObjectId peels every ref, which
         // is catastrophic on repos with many refs (e.g. Gerrit's refs/changes/*).
-        refsById = new HashMap<>();
+        Map<AnyObjectId, Set<Ref>> unsortedRefsById = new HashMap<>();
         RefDatabase refDb = repo.getRefDatabase();
         for (Ref ref : refDb.getRefsByPrefix(prefix)) {
           ObjectId target;
@@ -229,16 +229,17 @@
             target = ref.getObjectId();
           }
           if (target != null) {
-            refsById.computeIfAbsent(target, k -> new HashSet<>()).add(ref);
+            unsortedRefsById.computeIfAbsent(target, k -> new HashSet<>()).add(ref);
           }
         }
+        refsById = new HashMap<>();
+        for (Map.Entry<AnyObjectId, Set<Ref>> e : unsortedRefsById.entrySet()) {
+          refsById.put(
+              e.getKey(), e.getValue().stream().sorted(comparing(Ref::getName)).collect(toList()));
+        }
         refsByIdByPrefix.put(prefix, refsById);
       }
-      Set<Ref> refs = refsById.get(id);
-      if (refs == null) {
-        return ImmutableList.of();
-      }
-      return refs.stream().sorted(comparing(Ref::getName)).collect(toList());
+      return refsById.getOrDefault(id, ImmutableList.of());
     }
 
     private AbstractTreeIterator getTreeIterator(RevWalk walk, RevCommit commit)