In git aliases, shell commands are executed from the top-level directory
of the repo. Because of this, something like `git glog -- file` didn't
work in subdirectories.
Fix this by creating a wrapper script that first changes into
$GIT_PREFIX before executing the script. Since (currently) the only
use-case is to launch the functions in `autoload/git/`, most of the path
could move into the script, making the git config a lot cleaner.
