When enabled, older (non-latest) blessed API versions are stored as
`.gitref` files containing a `commit:path` reference instead of the full
JSON content. This allows Git to detect new API versions as renames.
(The format is designed to be fed into `git show`).

The git ref uses the first commit that introduced the file (not the
current merge-base) to ensure references remain stable as history
evolves.

Git ref storage is disabled by default. Enable with
`ManagedApis::with_git_ref_storage()`, or per-API with
`ManagedApi::use_git_ref_storage()`.

Demo in Omicron:

* oxidecomputer/omicron#9571
* oxidecomputer/omicron#9572

For more information, see [RFD
634](https://rfd.shared.oxide.computer/rfd/0634).