Provides a `source.git.ref` field for specifying a specific non-branch
non-tag git ref at which to perform a checkout. Providing one will
result in a shallow (depth 1) clone of the repo and shallow subsequent
fetches.

The motivation for this change is to support sourcing of trigger
templates from Gerrit patchsets. Gerrit uses non-branch/-tag refs
extensively and specifically in this case stores change patchsets in
`refs/changes/[slice]/[change]/[patchset]`. While arbitrary refs are
kind of an obscure git feature, there very well may be other git based
systems utilizing them.