[FIX] forwardport: apparently git/refs/heads can fuzzy-match

If the ref we asked for does not exist, github apparently decides to
fall-back to prefix-matching. So if we're trying to delete
already-deleted branch A and someone called their branch A-x we're
going to get it as a result.

Thankfully they were apparently smart enough to return a list even if
there's only a single fuzzy match. So if we get a list (instead of a
dict) as response to git/refs/heads assume the branch was already
deleted as if we got a 404.

forwardport/models/forwardport.py

@@ -140,6 +140,12 @@ class DeleteBranches(models.Model, Queue):
             return
 
         ref = ref.json()
+        if isinstance(ref, list):
+            _deleter.info(
+                "✘ got a fuzzy match (%s), branch probably deleted",
+                ', '.join(r['ref'] for r in ref)
+            )
+
         if ref['object']['sha'] != self.pr_id.head:
             _deleter.info(
                 "✘ branch %s head mismatch, expected %s, got %s",