Branch point/branch point tags

[Kyle Evans]

Hello!

For some context: I received an e-mail from the commit about
releng/13.0 being branched ("git: 638e531019fd - releng/13.0 - Branch
releng/13.0"), but it's unfortunately impossible to tell based on
e-mail context which commit it branched off on. Notably, commit races
happen and re@ isn't obligated to pick 'the latest at that exact time'
as far as I'm aware -- presumably they have carte blanche to pick
whichever commit they feel is a good choice.

We didn't really have this issue with svn because there was a single
commit that announced both implicitly in the addition summary as well
as explicitly which commit was chosen as the branch point (referencing
"svn commit: r339434 - stable/12").

We can of course discover what we care about in the git world by
clicking on the link to go to cgit and exploring the parent, but this
is decidedly not always convenient.

I had thought of one solution, but it's not great; adding the parent
commit hash to the mail when a new ref is pushed. That imposes some
weird limitations that remove some of the benefit of git, e.g., re@
would have to push the branch in the first commit then push any
follow-up unless we can slap it on 'the first commit in a push that
created a new ref'. This is kind of silly when they can just prepare
everything that needs to be done and push it with confidence in a
single step.

jhb@ pointed out on IRC another possible solution, and I'm wondering
what the git wg and re@ think... apparently, in the dark ages that far
predate me (CVS), we had something like a BP ("Branch Point") tag that
served the same purpose.

- Is this desirable information for anyone else to have?
- Would it screw up anything we actively try to do?

I imagine, for example, that re would just create an annotated
RELENG_13_0_BP tag and push that, which would serve as the
announcement that this is the commit for reference and perhaps also be
good bisect targets. While `git merge-base` works well for simply
matters, if you want to bisect from RELENG_13_0_BP to RELENG_13_1_BP
it's definitely quicker to conjure up the well-defined tag name than
`git bisect $(git merge-base releng/13.1 stable/13) $(git merge-base
releng/13.0 stable/13)`

Thoughts? Thanks,

Kyle Evans