Fail to merge if changes already exist in target

It's possible that the changes in an MR already exist in the target
branch - whether this was merged in manually or someone else happened to
have a similar idea. If this happens, when we rebase the MR on the
target the commits will be dropped, essentially turning into a no-op.
This causes the filter-branch to fail as it expects some revisions to be
listed. Whilst this has the same end result, it's clearer if we explain
exactly what the issue is.

marge/job.py

@@ -312,12 +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
+        if updated_sha == target_sha:
+            raise CannotMerge('these changes already exist in branch `{}`'.format(target_branch))
         if reviewers is not None:
             rewritten_sha = repo.tag_with_trailer(
                 trailer_name='Reviewed-by',
@@ -355,7 +358,6 @@ def update_from_target_branch_and_push(
 
         raise
     else:
-        target_sha = repo.get_commit_hash('origin/' + target_branch)
         return target_sha, updated_sha, rewritten_sha
     finally:
         # A failure to clean up probably means something is fucked with the git repo

tests/test_job.py

@@ -415,6 +415,20 @@ def test_waits_for_approvals(self, mock_log, unused_time_sleep):
         mock_log.debug.assert_any_call('Approvals haven\'t reset yet, sleeping for %s secs', ANY)
         assert api.state == 'merged'
 
+    def test_fails_if_changes_already_exist(self, unused_time_sleep):
+        api, mocklab = self.api, self.mocklab
+        expected_message = 'these changes already exist in branch `{}`'.format(
+            mocklab.merge_request_info['target_branch'],
+        )
+        with mocklab.expected_failure(expected_message):
+            job = self.make_job()
+            job.repo.rebase.return_value = mocklab.initial_master_sha
+            job.repo.get_commit_hash.return_value = mocklab.initial_master_sha
+            job.execute()
+
+        assert api.state == 'initial'
+        assert api.notes == ["I couldn't merge this branch: {}".format(expected_message)]
+
 
 class TestMergeJobOptions(object):
     def test_default(self):