Summary:
* Adds backup/restore forward/backward compatibility testing
* Adds forward/backward compatibility testing to sst ingestion
* More structure sharing and comments for the lists of branches
comprising each group
* Less reliant on invariants between groups with de-duplication logic
* Restructured for n+1 branch checkout+build steps rather than something
like 3n. Should be much faster despite more checks.

And to make manual runs easier

* On success, restores working trees to original working branch (aborts
early if uncommitted changes) and deletes temporary branch & remote
* Adds SHORT_TEST=1 mode that uses only the oldest version for each
* Adds USE_SSH=1 to use ssh instead of https for github
group

Test Plan: a number of manual tests, mostly with SHORT_TEST=1. Using one
version older for any of the groups (except I didn't check
db_backward_only_refs) fails. Changing default format_version to 5
(planned) without updating this script fails as it should, and passes
with appropriate update. TODO: full local run