Add merge_commit_sha and mergeable @properties to

pull_request.py:GitlabPullRequest.

'merge_commit_sha' maps exactly; however, in GitLab, the closest thing
to 'mergeable' appears to be 'merge_status', which is a string that
can take on values

- "unchecked"
- "checking"
- "can_be_merged"
- "cannot_be_merged"
- "cannot_be_merged_recheck"

For now, at least, we'll map "can_be_merged" to mergeable = True,
and everything else to False.

Signed-off-by: Ben Crocker <[email protected]>

ogr/services/gitlab/pull_request.py

@@ -103,6 +103,16 @@ def commits_url(self) -> str:
     def head_commit(self) -> str:
         return self._raw_pr.sha
 
+    @property
+    def merge_commit_sha(self) -> str:
+        return self._raw_pr.merge_commit_sha
+
+    @property
+    def mergeable(self) -> bool:
+        if self._raw_pr.merge_status == "can_be_merged":
+            return True
+        return False
+
     @property
     def source_project(self) -> "ogr_gitlab.GitlabProject":
         if self._source_project is None: