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>
1 file changed