upgrade-zulip-from-git: Check fetch refspecs, not mirror flag.
upgrade-zulip-from-git: Check fetch refspecs, not mirror flag.

While the `remote.origin.mirror` boolean being set is a very good
proxy for having been cloned with `--mirror`, is technically only used
when pushing into the remote[1].  What we care about is if fetches
from this remote will overwrite `refs/heads/`, or all of `refs/` --
the latter of which is most likely, from having run `git clone
--bare`.

Detect either of these fetch refspecs, and not the mirror flag.  We
let the upgrade process error out if `remote.origin.fetch` is unset,
as that represents an unexpected state.  We ignore failures to unset
the `remote.origin.mirror` flag, in case it is not set already.

[1]: https://git-scm.com/docs/git-config#Documentation/git-config.txt-remoteltnamegtmirror

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