Use default branch instead of master (#325)

marge/job.py

@@ -28,6 +28,10 @@ def __init__(self, *, api, user, project, repo, options):
     def repo(self):
         return self._repo
 
+    @property
+    def project(self):
+        return self._project
+
     @property
     def opts(self):
         return self._options
@@ -345,8 +349,8 @@ def update_from_target_branch_and_push(
             # A failure to clean up probably means something is fucked with the git repo
             # and likely explains any previous failure, so it will better to just
             # raise a GitError
-            if source_branch != 'master':
-                repo.checkout_branch('master')
+            if source_branch != self.project.default_branch:
+                repo.checkout_branch(self.project.default_branch)
                 repo.remove_branch(source_branch)
 
             if not branch_update_done:

marge/project.py

@@ -69,6 +69,10 @@ def project_seems_ok(project_info):
 
         return projects
 
+    @property
+    def default_branch(self):
+        return self.info['default_branch']
+
     @property
     def path_with_namespace(self):
         return self.info['path_with_namespace']

tests/test_project.py

@@ -10,6 +10,7 @@
     'path_with_namespace': 'cool/project',
     'ssh_url_to_repo': 'ssh://blah.com/cool/project.git',
     'merge_requests_enabled': True,
+    'default_branch': 'master',
     'only_allow_merge_if_pipeline_succeeds': True,
     'only_allow_merge_if_all_discussions_are_resolved': False,
     'permissions': {