bin/gitlab_gql: fix --print-merged-yaml when --rev != HEAD

Reading the local root config file and then asking gitlab to evaluate it
in the context of some other version will cause issues if they are not
identical.

Instead, the local document should be a simple include of whatever is
the root config file at that commit.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26074>

bin/ci/gitlab_gql.py

@@ -12,6 +12,7 @@ from itertools import accumulate
 from os import getenv
 from pathlib import Path
 from subprocess import check_output
+from textwrap import dedent
 from typing import Any, Iterable, Optional, Pattern, TypedDict, Union
 
 import yaml
@@ -349,9 +350,10 @@ def print_dag(dag: Dag) -> None:
 
 
 def fetch_merged_yaml(gl_gql: GitlabGQL, params) -> dict[str, Any]:
-    gitlab_yml_file = get_project_root_dir() / ".gitlab-ci.yml"
-    content = Path(gitlab_yml_file).read_text().strip()
-    params["content"] = content
+    params["content"] = dedent("""\
+    include:
+      - local: .gitlab-ci.yml
+    """)
     raw_response = gl_gql.query("job_details.gql", params)
     if merged_yaml := raw_response["ciConfig"]["mergedYaml"]:
         return yaml.safe_load(merged_yaml)