Silence 'Current branch %s is up to date' during sync

We accidentally introduced this message during 1.6.8 by always
invoking `git rebase` when there were no new commits from the
upstream, but the user had local commits.

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

project.py

@@ -733,9 +733,7 @@ class Project(object):
         last_mine = commit_id
         cnt_mine += 1
 
-    if not local_changes and not upstream_gain:
-      # Trivially no changes caused by the upstream.
-      #
+    if not upstream_gain and cnt_mine == len(local_changes):
       return
 
     if self.IsDirty(consider_untracked=False):