Do not retain commit body during bitmap generation

The bitmap preparer only needs commit graph topology; it does not use
the message body.  Allow the RevWalk to free the body after the commit
has been parsed to save memory.

Change-Id: I97d4a440c9fc313873fd224bd05b9d9e3dc575db

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

@@ -114,6 +114,7 @@ class PackWriterBitmapPreparer {
 			IOException {
 		pm.beginTask(JGitText.get().selectingCommits, ProgressMonitor.UNKNOWN);
 		RevWalk rw = new RevWalk(reader);
+		rw.setRetainBody(false);
 		WalkResult result = findPaths(rw, expectedNumCommits);
 		pm.endTask();