Only throw MissingObjectException when necessary

When preparing the bitmap, the flag ignoreMissingStart only applied to
the start object. However, sometime the start object is present but some
related objects are not present during the walk, we should only release
the MissingObjectException when the ignoreMissingStart is set false.

Change-Id: I1097a2defa4a9dcf502ca8baca5d32880378818f
Signed-off-by: Zhen Chen <czhen@google.com>

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

@@ -117,6 +117,7 @@ final class PackWriterBitmapWalker {
 						new AddUnseenToBitmapFilter(seen, bitmapResult));
 			}
 
+			try {
 				while (walker.next() != null) {
 					// Iterate through all of the commits. The BitmapRevFilter does
 					// the work.
@@ -136,6 +137,20 @@ final class PackWriterBitmapWalker {
 					bitmapResult.addObject(ro, ro.getType());
 					pm.update(1);
 				}
+			} catch (MissingObjectException e) {
+				if (!ignoreMissingStart) {
+					throw e;
+				}
+				// Even when none of the objects we started the walk from is missing,
+				// an object reachable from one can be. RevWalk and ObjectWalk don't
+				// provide a way to ignore the missing object and continue, so bail
+				// out early with an undersized bitmap.
+				//
+				// The resulting packfile is likely to be much too large, but that's
+				// better than serving an error.
+				//
+				// TODO(czhen): Resume the walk instead once RevWalk supports that.
+			}
 		}
 
 		return bitmapResult;