[1/n] add git ref storage for older API versions

[sunshowers]

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:

• demo showing git refs omicron#9571
• demo showing adding a new sled-agent API revision omicron#9572

For more information, see RFD 634.

TODO:

• add note to the commit message about how this makes git treat the file as a rename
• handle situation where multiple new versions are added in a single commit
• allow per-version customization of gitref storage (useful when particular fixed versions are special in some way) will do this when we need it
• verify that adding a version, then removing it, then adding it again chooses the latest version
• automated + manual tests around merge conflicts (see [2/n] handle unparseable local files (e.g., conflict markers) #39)
• documentation

[david-crespo]

Wow, that is awesome. Didn’t know you can do that. So the gitref effectively takes the old file out of the current list of files, allowing git to treat the change as a move?

Edit: discussed in chat. Answer is yes.