Remove unneeded packs when compacting with no new objects

Previously, the DfsPackCompactor exited without pruning the existing
packs, when no new packs were created.

Change-Id: I5e3b6f8c789706c7a982e6ae93cf7c3d4346797c

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsPackCompactor.java

@@ -220,8 +220,14 @@ public class DfsPackCompactor {
 				pw.setReuseDeltaCommits(false);
 
 				addObjectsToPack(pw, ctx, pm);
-				if (pw.getObjectCount() == 0)
+				if (pw.getObjectCount() == 0) {
+					List<DfsPackDescription> remove = toPrune();
+					if (remove.size() > 0)
+						objdb.commitPack(
+								Collections.<DfsPackDescription>emptyList(),
+								remove);
 					return;
+				}
 
 				boolean rollback = true;
 				DfsPackDescription pack = objdb.newPack(COMPACT);