git.latest: Support shallow-cloning tags
git.latest: Support shallow-cloning tags

For long, we've supported shallow clones of branches only, since git-clone
supports only a '--branch' option but no '--tag' options.

Turns out, the same '--branch' option supports tag names since Git
1.8.0 (released Oct 2012) [1], so it's a good idea for us to support it too.

This adds support for specifying a tag in the 'rev' option when 'depth' is used
with 'git.latest'.

1: https://git-scm.com/docs/git-clone/1.8.0#git-clone---branchltnamegt

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt