This implements the most basic workflow for submitting changes to Gerrit,
through a verb called 'upload'. This verb is chosen to match the gerrit
documentation (https://gerrit-documentation.storage.googleapis.com/Documentation/3.12.1/user-upload.html).
Given a list of revsets (specified by multiple `-r` options), this will parse
the footers of every commit, collect them, insert a `Change-Id` based off the jj change-id (if one doesn't already exist), and then push them into the given remote.
It will then abandon the transaction, thus ensuring that gerrit change-IDs do not appear in jj.
Because the argument is a revset, you may submit entire trees of changes at
once, including multiple trees of independent changes, e.g.
jj gerrit upload -r foo:: -r baz::
Signed-off-by: Austin Seipp <aseipp@pobox.com>
