Delete a cached Git repository from the UI and API #685

Follow-up to #675, which lists cached Git repositories on /ui/git.

Today the only way to remove one repository from the git cache is to stop the server and delete its directory under git.cache_dir. Deleting it while the server runs leaves the cache's size accounting wrong until the next start and can break a running fetch.

Proposal, behind the existing cache_delete setting:

• DELETE /api/git/{upstream}/{path}.git removes one cached repository, and /ui/git gets a Delete button per row, sending to /ui/api/git/... like the version page does for artifacts.
• It reuses the eviction path: the repository is dropped from the cache at once, so the next request clones it again, and its directory is removed once no request is reading it.
• A repository in the middle of a clone or fetch answers 409 rather than waiting.
• The response says what was removed and how many bytes are freed.

I'd open the PR after #675 is in. Does this fit, or would you rather keep git out of cache_delete?