Validate Git history continuity in CI stages

[pkgw]

Upon apply-versions, check:

• That current origin rc tip is an ancestor of new rc tip
• That current origin release main-dev commit is an ancestor of new rc main-dev commit

(where "main-dev" commit is the commit from the main-dev branch used as the direct basis for the rc and release commits ... which we can discover by looking at the rc and release commit parents, knowing how those branches are created.)

[pkgw]

Also, upon confirm, check that origin rc main-dev commit is an ancestor of HEAD, I think?