git mirrors commit only fetch

The [git mirrors experiment fetch/checkout process](https://github.com/buildkite/agent/blob/9c399b6e987da50d61452b61318dd1617261ea02/bootstrap/bootstrap.go#L1015-L1091) doesn't currently support commit-only fetch, like [the regular fetch/checkout process](https://github.com/buildkite/agent/blob/9c399b6e987da50d61452b61318dd1617261ea02/bootstrap/bootstrap.go#L1185). This is a great way to fetch only the commit under test as an optimisation, and also sometimes is able to fetch more commits than can be seen by a bare ref, including those that have been force pushed over, or from forked repositories.

Start in bootstrap/bootstrap.go, comparing the git mirrors fetch/checkout process around lines 1015-1091 with the regular fetch/checkout process around line 1185. Trace how the commit under test is selected and fetched, then verify that mirror-based jobs can fetch only that commit while retaining the documented optimization and reachability behavior.

Written by the indexing model from the issue text.