[Tracking] Rebasing Against Upstream

[RandomDSdevel]

A detailed description of the proposed feature

     Rebase against upstream, pending intended upcoming renewed activity here downstream. Namely:

1. Open a PR to rebase brew.rb: Undeprecate HOMEBREW_BUILD_FROM_SOURCE #5, currently our only change beyond/against upstream. (Done: I opened [WIP] Manually 'Rebase with History' and Reapply #5 #7.)
2. Update all other branches to rebase them against upstream changes.
3. Merge [WIP] Manually 'Rebase with History' and Reapply #5 #7.

The motivation for the feature

     This fork isn't dead, not if I have anything to say about it. It's only mostly dead, and, as the saying goes, that's also…slightly alive! (To paraphrase another relevant quip, "Any rumors of this fork's death have been greatly exaggerated.")

How the feature would be relevant to at least 90% of Portage-Brew users

     Further work here remains desirable for all of this fork's users.

What alternatives to the feature have been considered

     No alternatives considered.