For installs which use the `upgrade-zulip-from-git` process, the
deployment directory is a git checkout. This means that an
administrator can, as an emergency tool, run `git revert` and similar
commands -- assuming there is a `~/.gitconfig` set up for the zulip
user.
Add commands to `scripts/lib/install` to create a `~/.gitconfig` file
at installation time. The `user.name` and `user.email` fields are set
to the hostname and passed-in `--email` value, respectively.
Fixes#18039.
d2706fa246