Fix 'repo sync' when the remote reflog has only 1 entry

If the reflog for the upstream branch has only 1 entry in it, as
the branch has been updated only once, we can get back the 0{40}
object id from `git rev-parse upstream@{1}`, in which case we should
consider it to be the same as if upstream@{1} is not defined.

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

project.py

@@ -517,6 +517,9 @@ class Project(object):
         old_merge = self.bare_git.rev_parse('%s@{1}' % merge)
       except GitError:
         old_merge = merge
+      if old_merge == '0000000000000000000000000000000000000000' \
+         or old_merge == '':
+        old_merge = merge
     else:
       # The upstream switched on us.  Time to cross our fingers
       # and pray that the old upstream also wasn't in the habit