Use the .mergify.yml from PR

When a PR on the default branch change .mergify.yml, we must
use that one instead. So configuration and testing will report
useful information.

Closes #24

mergify_engine/engine.py

@@ -112,8 +112,7 @@ def handle(self, event_type, data):
         # BRANCH CONFIGURATION CHECKING
         branch_rule = None
         try:
-            branch_rule = rules.get_branch_rule(
-                self._r, incoming_pull.base.ref)
+            branch_rule = rules.get_branch_rule(self._r, incoming_pull)
         except rules.NoRules as e:
             # Not configured, post status check with the error message
             incoming_pull.mergify_engine_github_post_check_status(

mergify_engine/rules.py

@@ -91,12 +91,21 @@ def dict_merge(dct, merge_dct):
             dct[k] = merge_dct[k]
 
 
-def get_branch_rule(g_repo, branch):
-    # TODO(sileht): Ensure the file is valid
+def get_branch_rule(g_repo, incoming_pull):
     rule = copy.deepcopy(DEFAULT_RULE)
 
+    branch = incoming_pull.base.ref
+
+    ref = github.GithubObject.NotSet
+    if g_repo.default_branch == branch:
+        # NOTE(sileht): If the PR on the default branch change the .mergify.yml
+        # we use it, otherwise we the file on the default branch
+        for f in incoming_pull.get_files():
+            if f.filename == ".mergify.yml":
+                ref = f.contents_url.split("?ref=")[1]
+
     try:
-        content = g_repo.get_contents(".mergify.yml").decoded_content
+        content = g_repo.get_contents(".mergify.yml", ref=ref).decoded_content
         LOG.info("found mergify.yml")
     except github.UnknownObjectException:
         raise NoRules(".mergify.yml is missing")