Trigger to update git k5login file

[Ken Hornstein]

In my general quest to make Kerberos authentication work cleaner with
gitolite, I wrote a trigger script to automatically iterate over the
list of Kerberos principals specified in the gitolite configuration
and generate an appropriate ".k5login" file (previously in our setup
we had to manually add a user to the .k5login file for the git account
every time we added a new user to the repo). I've attached this trigger
script in the hopes that other people might find it useful.

Feedback is welcome!

--Ken