Git is amazing. Specifically, its ability to be extended is amazing. Essentially any "git <command>" can be made by naming a file "git-<command>" and placing it in your git-core folder. Let's look at an example.
Assume you are writing a script to give you a list of all stashes and print out the diff of each staff, all in one fell swoop. Assume you want to access this script by typing "git stash-list".
First, name the file "git-stash-list". Then, put this file in your git-core folder. You can find this folder by doing the following:
$ find / -name "git-core"
I'm on OS X 10.6 and found it in /opt/local/libexec/git-core and I believe I'm using the MacPorts version. Then, be sure to chown and chmod the file the same as the other "git-*" commands in that directory. For me, I did:
$ sudo chown root:admin git-stashlist $ sudo chmod 755 git-stashlist