test for presence of .git folders for pull/clone

To decide between doing a git clone or a git pull, test for the existence of the .git folder inside the target destination, not just the existence of the target folder itself.

This allows for a peaceful coexistance between libretro_fetch.sh and git submodules to come.
Merge pull request #103 from l3iggs/patch-1

test for presence of .git folders for pull/clone