Add new "relative" option to cwd_truncate

[olivierdagenais]

Overview

This keeps the window title shorter: once I'm in a Git repo, I only care about the repo folder's name. It also makes sure the "shell label" uses the same value of the computed cwd.

Manual testing

This is how I use it in my ~/.git-prompt.conf file:

cwd_cmd='cwd_truncate relative'

...and it looks like this:

Mission accomplished!