ci_run_n_monitor: allow detached heads as well

When running on a detached head (eg. checkout of a tag or a specific
commit), there is no active branch, so we can't perform this check; just
skip it and assume the user knows what they're doing.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27578>

bin/ci/ci_run_n_monitor.py

@@ -20,7 +20,7 @@ from collections import defaultdict
 from concurrent.futures import ThreadPoolExecutor
 from functools import partial
 from itertools import chain
-from subprocess import check_output
+from subprocess import check_output, CalledProcessError
 from typing import TYPE_CHECKING, Iterable, Literal, Optional
 
 import gitlab
@@ -426,10 +426,15 @@ if __name__ == "__main__":
                 REV = check_output(['git', 'rev-parse', REV]).decode('ascii').strip()
 
                 if args.rev == 'HEAD':
+                    try:
                         branch_name = check_output([
                             'git', 'symbolic-ref', '-q', 'HEAD',
                         ]).decode('ascii').strip()
+                    except CalledProcessError:
+                        branch_name = ""
 
+                    # Ignore detached heads
+                    if branch_name:
                         tracked_remote = check_output([
                             'git', 'for-each-ref', '--format=%(upstream)',
                             branch_name,