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>
diff --git a/project.py b/project.py
index 45229e5..1beee2a 100644
--- a/project.py
+++ b/project.py
@@ -733,9 +733,7 @@
         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):