Take squashed MRs into account

When a MR is merged via WebUI using the 'squash commits' option the
squashed commit is assigned to the 'squash_commit_sha' property of the
MR object. We have to test for this property too, when looking which MR
a commit belongs to in case the pipeline runs on a squashed commit.
Otherwise parent MR cannot be found.

get_merge_requests.py

@@ -26,7 +26,7 @@ def get_merge_requests(
         )
     if commit:
         for mr in all_merge_requests:
-            if mr.sha == commit:
+            if mr.sha == commit or mr.squash_commit_sha == commit:
                 merge_requests.append(mr)
     elif all_merge_requests:
         merge_requests = all_merge_requests