This ensures that the next `upgrade-zulip-from-git` has access to the
commit history of the initial install, if it was from a forked
repository. `/home/zulip/deployments/current` and `/srv/zulip.git`
are not quite organized into the steady-state that they will have
after one `upgrade-zulip-from-git`:
- `/home/zulip/deployments/current` is its own clone, not a worktree
- `/srv/zulip.git` has an origin of `/home/zulip/deployments/current`
- `remote.origin.mirror` is set on `/srv/zulip.git`
- `remote.origin.fetch` is `+refs/*:refs/*`
All but the first are automatically cleaned up by
`upgrade-zulip-from-git` when it is next run, using the code added in
30457ecd02. The additional complexity of making an existing
independent clone into a worktree seem not worth solving the first
point.
ec4bfd7c78