git: <git-checkout> allows tags in its 'commit' field.

Fixes <https://bugs.gnu.org/36371>.
Reported by Tobias Geerinckx-Rice <me@tobias.gr>.

* guix/git.scm (git-checkout-compiler): Pass 'tag-or-commit' to
'latest-repository-commit*'.
* doc/guix.texi (Package Transformation Options): Update '--with-commit'
documentation accordingly.
* tests/guix-build-branch.sh: Add test.