Makefile: Hack around `git subtree` brokenness
Makefile: Hack around `git subtree` brokenness

Problem background:

 1 Because of regressions in Git 2.20, `git subtree split` can get
   confused about whether a commit is part of the already-split-off
   subtree¸or is a commit that predates the subtree.
 2 Because 'docs/docs/' exists, without excessive `git-subtree-split:` or
   `--onto=` hints, `git subtree split` can get confused about whether a
   commit is part of the already-split-off subtree, or is a commit in
   mainline that includes the subtree.
 3 Because `git subtree merge` doesn't add the rejoin metadata that
   `git subtree split --rejoin` does, _every_time_ you `git subtree split`,
   it has to re-do all of the work where it makes the above mistakes.
   Also, that just means that splits are slow.

Problem result:

 - `make push-docs` fails because it's a non-fast-forward push.

Fix:

 A Just accept (1) as a mistake that happened when I did the initial
   `git subtree split`.  It's in the git history now.
 B Mitigate (2) and preserve (1) by passing
   `--onto={ambassador-docs#master}` to `git subtree split`.  This
   gives it enough information to avoid making mistake (2).  It also gives
   records it the wrong decisions it made because of (1), so that it will
   continue to make consistent history even if/when but (1) gets fixed.
 C Have `make pull-docs` immediately do a `split --rejoin`, in order to
   add the join metadata.  This should cause `make push-docs` to be
   faster.  This should cause (B) to eventually bu unnecessary.

datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador