We currently ensure this by simply overriding any existing template.
This is of course sub-optimal, but the previous recommendation, to
globally set `commit.template` to a relative path would otherwise make
`git commit` crash when run in a repo without first creating that
template file, e.g. by running `git mob ...` or `git solo` in the
project directory before committing anything.
This way, we don't have to globally set `commit.template`, and git-mob
will still work.
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.