Since the only thing the `repo` argument to `with_repo_or_exit` was used
for, was to extract the gitmessage template path from it, we can just do
that directly instead.
It can:
* Parse the `~/.git-coauthors` file
* List available co-authors with --list
* Filter available co-authors given a list of initials, giving an error
if no co-author matches a given initial
Martin Frost