git_operations: simplify metadata-fetch options and reuse test helper

Cleanups from /simplify review (no behavior change):

• Collapse fetchMetadataOpts to a single noFilter bool. After dropping the Shallow field, the struct held only NoFilter plus a dead Unshallow field (never set true by any caller); the two named wrappers FetchMetadataBranch / FetchMetadataTreeOnly document intent at the only two call sites.
• Reuse the existing gitOutput test helper instead of the duplicate gitOut, and drop the thin gitRevParse wrapper.