oss-fuzz: set a gitdir in the commit-graph fuzzer

Since f1141b4391 (commit-graph: refactor `parse_commit_graph()` to take
a repository, 2025-08-15), parse_commit_graph() calls
prepare_repo_settings().

The fuzzer sets settings.initialized, so that git reads no
configuration. But prepare_repo_settings() checks for a gitdir before
it reads that flag, and the repository of the fuzzer has no gitdir.
Thus each input that passes the header and the three required chunks
stops at

    BUG: repo-settings.c:40: Cannot add settings for uninitialized repository

The fuzzer never reaches the code that reads the optional chunks:
generation data and changed-path Bloom filters.

Set a placeholder gitdir, as test-path-utils does.
prepare_repo_settings() checks only that the gitdir is not NULL. It
then returns, because the fuzzer marks the settings as initialized.
repo_clear() frees the gitdir, and no code opens a file under it.

Signed-off-by: Sami Jawhar <sami@theorem.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

oss-fuzz/fuzz-commit-graph.c

@@ -20,6 +20,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 	 * possible.
 	 */
 	repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+	the_repository->gitdir = xstrdup(".git");
 	the_repository->settings.initialized = 1;
 	the_repository->settings.commit_graph_generation_version = 2;
 	the_repository->settings.commit_graph_changed_paths_version = 1;