upgrade-zulip-from-git: init, then add remote.
upgrade-zulip-from-git: init, then add remote.

30457ecd02b5db0a468dee8dc3953a9af8d82ff5 removed the `--mirror` from
initial clones, but did not add back `--bare`, which `--mirror`
implies.  This leads to `/srv/zulip.git` having a working tree in it,
with a `/srv/zulip.git/.git` directory.

This is mostly harmless, and since the bug was recent, not worth
introducing additional complexity into the upgrade process to handle.

Calling `git clone --bare`, however, would clone the refs into
`refs/heads/`, not the `refs/remotes/origin/` we want.  Instead, use
`git init --bare`, followed by `git remote add origin`.  The remote
will be fetched by the usual `git fetch --all --prune` which is below.

andersk/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip