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

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8012

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. Full local run passed (had to remove "2.7.fb.branch"
due to compiler issues, also before this change).

Reviewed By: riversand963

Differential Revision: D26735840

Pulled By: pdillinger

fbshipit-source-id: 1320c22de5674760657e385aa42df9fade8b6fff