Ignore dangling warning in stale_refs

If pruning a remote ref git outputs a warning if this would make
another ref dangling. We're not interested in this information in
stale_refs so ignore this case.

git/remote.py

@@ -742,6 +742,10 @@ def stale_refs(self) -> IterableList[Reference]:
             # expecting
             # * [would prune] origin/new_branch
             token = " * [would prune] "
+            # Valid output also can be
+            # <ref> will become dangling!
+            if 'will become dangling!' in line:
+                continue
             if not line.startswith(token):
                 continue
             ref_name = line.replace(token, "")