Fetch target sha after fusing

A side effect of merging/rebasing is that we re-fetch our remotes
(origin and source). This affects the target sha as it looks at the
locally cloned repo, and doing this before we fetch means it will be
outdated.

marge/job.py

@@ -312,13 +312,15 @@ def update_from_target_branch_and_push(
     branch_updated = branch_rewritten = changes_pushed = False
     try:
         fuse = repo.merge if use_merge_strategy else repo.rebase
-        target_sha = repo.get_commit_hash('origin/' + target_branch)
         rewritten_sha = updated_sha = fuse(
             branch=source_branch,
             new_base=target_branch,
             source_repo_url=source_repo_url
         )
         branch_updated = True
+        # The fuse above fetches origin again, so we are now safe to fetch
+        # the sha from the remote target branch.
+        target_sha = repo.get_commit_hash('origin/' + target_branch)
         if updated_sha == target_sha:
             raise CannotMerge('these changes already exist in branch `{}`'.format(target_branch))
         if reviewers is not None: