Add progress monitor step to first phase of GC commit selection

Change-Id: I10e3d53de5e2b1fc1be9a59b236da5780473d841
Signed-off-by: Terry Parker <tparker@google.com>

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

@@ -452,6 +452,7 @@ class PackWriterBitmapPreparer {
 		RevCommit rc;
 		while ((rc = rw.next()) != null && pos > 0) {
 			commits[--pos] = rc;
+			pm.update(1);
 		}
 
 		// Sort the new wants by reverse commit time.