Support partial pulls and respect GIT_SSH
Support partial pulls and respect GIT_SSH

First, allow pulling individual files with the following syntax:

    $ git fat pull foo.png bar.png bat.png
    $ git fat pull rsync foo.png bar.png bat.png

Second, use GIT_SSH for rsync transport. If the user has configured git to
use a particular ssh command by setting the environment variable `GIT_SSH`,
then use that command as the rsync shell.

See also git's documentation on this environment variable:
http://git-scm.com/book/en/v2/Git-Internals-Environment-Variables#Miscellaneous

cyaninc/git-fat,ciena-blueplanet/git-fat,ciena-blueplanet/git-fat,ajohnson23/git-fat,cyaninc/git-fat,lpsinger/git-fat,ajohnson23/git-fat,lpsinger/git-fat