Fix GC all-branches test on CI (explicit \git init -b main\)

GitHub runners' git defaults to \`master\` for new repos if no
init.defaultBranch is configured. Use explicit \`-b main\` to make
\`git checkout main\` work across environments.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ryan-williams

tests/test_gc.py

@@ -248,7 +248,7 @@ def test_compute_gc_plan_all_branches(tmp_path):
     repo = tmp_path / "repo"
     repo.mkdir()
 
-    subprocess.run(["git", "init"], cwd=repo, capture_output=True, check=True)
+    subprocess.run(["git", "init", "-b", "main"], cwd=repo, capture_output=True, check=True)
     subprocess.run(["git", "config", "user.email", "t@t.com"], cwd=repo, capture_output=True, check=True)
     subprocess.run(["git", "config", "user.name", "T"], cwd=repo, capture_output=True, check=True)
     (repo / ".dvc").mkdir()