This ensures that the `git describe` queries that we run for caching
Zulip's Git version are guaranteed to include recent releases.
This change ensures that we have accurate output even if we're pointed
at a fork of Zulip that never updates its tags.
Additionally, it will make it possible to record the
`git merge-base upstream/master` in future commits.
Note that because we run this code before unpacking the new version,
the pre-upgrade version of this code runs.
As a result, we cannot assume that the upstream repository exists.
03420831b0