backport --revert: Use gerrit_push_user to push revert commit to Gerrit

Use with_env_vars_set_for_user() when creating the revert commit, to
ensure that author and committer name and email address are configured
to reasonable values (basically Gecos user@deploy-server-fqdn).
This avoids a potential warning message from git.

Use gerrit_push_user and gerrit_push_url to push the revert commit to
Gerrit using SSH instead of attempting to push to Gerrit via HTTP.
This removes the need for users to populate a .netrc file in their
home directory.

Bug: T366217

---

Related MRs:

• !338 (merged)
• !344 (merged)
• !341 (merged)
• !340 (merged) (This MR)
• master