Add an update option to the git module

This will allow just making sure the repo exists w/out actually making
any chnages to it. Useful if you're going to run a play repeatedly
against a host that might have active development going on in the
repository (think initial bootstrap of a developers system, and
continued playbook runs to work on other projects).
Merge pull request #2934 from j2sol/git-update

Add an update option to the git module