Skip redundant 'OR-reuse' step in tip commit bitmap setup

When creating bitmaps during gc, the bitmaps in tipCommitBitmaps are
built in setupTipCommitBitmaps using the following procedure:

 0. create a bitmap ('reuse') that lists all ancestors of commits
    whose existing bitmaps will be reused.  I will call this the
    reused part of history.

 1. initialize a bitmap for each of the pack's "want"s by taking
    a copy of the 'reuse' bitmap and setting the bit corresponding
    to the one wanted commit.

 2. walk through ancestors of wants, excluding the reused part of
    history.  Add parents of visited commits to bitmaps that have
    those commits.

 3. AND-NOT each tipCommitBitmap against the 'reuse' bitmap

 4. Sort the bitmaps and AND-NOT each against the previous so they
    partition the new commits.

The OR against 'reuse' in step 1 and the AND-NOT against 'reuse'
cancel each other out, except when commits from the reused part of
history are added to a bitmap in step 2.  So avoid adding commits from
the reused part of history in step 2 and skip the OR and AND-NOT.

Performance impact (thanks to Terry for measuring):

The initial "selecting bitmaps" phase of garbage collection decreased
from (83 + 81 + 85) / 3 = 83 to (56 + 57 + 56) / 3 = 56.3, meaning
nearly a ~50% speedup of that phase.

Tested-by: Terry Parker <tparker@google.com>
Change-Id: I26ea695809594347575d14a1d8e6721b8608eb9c

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriterBitmapPreparer.java

@@ -410,7 +410,6 @@ class PackWriterBitmapPreparer {
 			rw.markStart(rc);
 
 			BitmapBuilder bitmap = commitBitmapIndex.newBitmapBuilder();
-			bitmap.or(reuse);
 			bitmap.addObject(rc, Constants.OBJ_COMMIT);
 			tipCommitBitmaps.add(new BitmapBuilderEntry(rc, bitmap));
 		}
@@ -430,19 +429,15 @@ class PackWriterBitmapPreparer {
 					continue;
 				}
 				for (RevCommit c : rc.getParents()) {
+					if (reuse.contains(c)) {
+						continue;
+					}
 					bitmap.addObject(c, Constants.OBJ_COMMIT);
 				}
 			}
 			pm.update(1);
 		}
 
-		// Remove the reused bitmaps from the tip commit bitmaps
-		if (!reuseCommits.isEmpty()) {
-			for (BitmapBuilderEntry entry : tipCommitBitmaps) {
-				entry.getBuilder().andNot(reuse);
-			}
-		}
-
 		// Sort the tip commit bitmaps. Find the one containing the most
 		// commits, remove those commits from the remaining bitmaps, resort and
 		// repeat.