PackBitmapIndex: Set distance threshold

Setting the distance threshold to 2000 in PackWriterBitmapPreparer to
reduce memory usage in garbage collection. When the threshold is 0, GC
for the msm repository would use about 37 GB memory to complete. After
setting it to 2000, GC can finish in 75 min with about 10 GB memory.

Change-Id: I39783eeecbae58261c883735499e61ee1cac75fe
Signed-off-by: Yunjie Li <yunjieli@google.com>

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

@@ -58,7 +58,7 @@ class PackWriterBitmapPreparer {
 
 	private static final int DAY_IN_SECONDS = 24 * 60 * 60;
 
-	private static final int DISTANCE_THRESHOLD = 0;
+	private static final int DISTANCE_THRESHOLD = 2000;
 
 	private static final Comparator<RevCommit> ORDER_BY_REVERSE_TIMESTAMP = (
 			RevCommit a, RevCommit b) -> Integer