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.
`git-solo` clears the `.git/.gitmessage` commit message template in the
current git repository.
If the current working directory is not within a git repository, it will
print `Not in a git repository` on stderr and exit.