Switch to using -C instead of --git-dir

[Dave Reid]

From https://bitbucket.org/davereid/drush-patchfile/pull-requests/10/git-dir-option-must-be-present-before-the/diff:

I wonder if we could switch to using -C now that it's available in Git 1.8.5: http://stackoverflow.com/a/20115678 (and has been out since 2013).