Gerrit also needs to be able to push low-level refs into upstream remotes, just
like `jj git push` does. Doing so requires providing callbacks e.g. for various
password entry mechanisms, which was private to the `git` command module.
Pull these out to a new module `git_utils` so we can reuse it across the two
call sites.
This also moves a few other strictly Git-related functions into `git_utils`
as well, just for the sake of consistency.
Signed-off-by: Austin Seipp <aseipp@pobox.com>