Make it possible to clone a specific ref

The GitLab CI runners want to fetch and check out a specific ref to run the tests against.

Currently it does something like this:

git init repo
git -C repo remote add origin path/to/repo.git
git -C repo fetch origin refs/merge-requests/6509/merge
git switch --detach FETCH_HEAD

It doesn't matter whether the GIT_STRATEGY is fetch or clone, in both cases git-fetch(1) is used, because git-clone(1) simply doesn't support cloning a single ref. git-clone(1) has option --single-branch and --branch=<branch-name>, but it's not possible specify a non-branch ref.

Proposal

Add single ref cloning support to git-clone(1). The correct design of the name and format of arguments is still to be decided.