Feature: Use $HOME instead of /home/username in ~/.gitconfig

Hi,

Currently the `[maintenance]` section of `~/.gitconfig` has something
like:

[maintenance]
    repo = /home/username/.znap/repos/asdf

I'm sharing my `.dotfiles` across systems, so I'd prefer
`/home/username` be replaced with `$HOME` or even with `~/`.

[maintenance]
    repo = $HOME/.znap/repos/asdf

I'd be happy to make a patch, if somebody let me know where, in this
huge repository: https://github.com/git/git

Thanks,
Joe