Fix clearing of topic during replace

When uploading a replacement patch set to a change that already has
a topic tag, we don't want to clear the topic if the user hasn't
specified it on the command line.

Bug: issue 676
Change-Id: I89c6b7290dab7bbf37c15cad11004fa886e80a7e
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed