Add check mode to git module

Related to issue #2114.  This hooks in check_mode to git module.  This
will exit with changed=True at the following places:

* If the repo has not been cloned
* If the destination has local modifications, this will exit with
  changed=True.  This is because reset() will exit anyways if there are
  local mods and force is False.
* If the remote HEAD commit id is not the same as that of the local HEAD.
  This is determined by get_remote_head() that runs 'git ls-remote' to
  determine remote HEAD.

Lastly, if this is run with check_mode enabled, this will exit with
changed=False before fetch() is invoked so that no local mods are made.

Comments are welcome.  :-)