fetch: select a valid branch to workaround a gix bug

Author: chenx97

acbs/fetch.py

@@ -143,8 +143,10 @@ def gix_fetch(info: ACBSSourceInfo, source_location: str, name: str) -> Optional
     else:
         logging.info('Updating repository with gix...')
         # gix doesn't have the --prune option yet.
-        # FIXME: reset HEAD to master to fix gix can't second fetch src issue.
-        subprocess.check_call(['git', 'symbolic-ref', 'HEAD'], cwd=full_path)
+        # FIXME: reset HEAD to the first valid branch to fix gix can't second fetch src issue.
+        subprocess.check_call(['git', 'symbolic-ref', 'HEAD', 'refs/heads/HEAD'], cwd=full_path)
+        valid_branches = subprocess.check_output(['git', 'branch'], cwd=full_path)
+        subprocess.check_call(['git', 'symbolic-ref', 'HEAD', 'refs/heads/' + valid_branches.splitlines[0]], cwd=full_path)
         os.remove(os.path.join(full_path, "index"))
         subprocess.check_call(
             ['gix', 'fetch', 'origin', '+refs/heads/*:refs/heads/*'], cwd=full_path)