Fast-forward a fully merged topic branch during 'repo sync'

Instead of trying to rebase the changes on a topic branch that
has been fully merged into the upstream branch we track, we should
just fast-forward the topic branch to the new upstream revision.
This way the branch doesn't try to rewrite commits that are already
merged in the upstream.

Signed-off-by: Shawn O. Pearce <sop@google.com>

project.py

@@ -563,6 +563,15 @@ class Project(object):
           _info("[%s] Consider merging or rebasing the"
                 " unpublished commits.", self.name)
         return True
+      else:
+        # We can fast-forward safely.
+        #
+        try:
+          self._FastForward(rev)
+        except GitError:
+          return False
+        self._CopyFiles()
+        return True
 
     if merge == rev:
       try: