Create central branch or tag that points revision referred by mozilla-central

[arai-a]

Related to #563
we should have easy way to checkout a pair of jsparagus/mozilla-central revisions that are known to work.

we could create an automation that updates central branch or tag that points jsparagus revision in master branch, that is known to work.
and store the mozilla-central revision hash somewhere.

[arai-a]

this could be done as a part of github action in #569

[nbp]

Note, when we have to apply additional changes to m-c, we might not have a valid hash for mozilla-central. Instead, we might have a base-hash and a series of patches to apply on top.

[arai-a]

#570 is going to track the known good revisions pair for mozilla-central + jsparagus.

for jsparagus side, we can add branch or tag that point it.
and for mozilla-central side, we can add smoosh_tools command that shows (or checkouts) the revision.
so that there's no need to handle not-yet-landed patches when building smooshmonkey locally