So far we relied on a hack in our Jenkins job (set_origin_base_branch_head) to
tweak Git state in a way that git_base_branch_head gets it right. But that's
complex, fragile, and interacts badly with the refs mangling we do in
build-tails. So let's instead honor a pre-existing $BASE_BRANCH_GIT_COMMIT, so
we can drop the set_origin_base_branch_head hack from the Jenkins job.

What I'm doing here seems to have been the original intent of the work that was
done on #11972: f4a20a41 and
49c29304 have started work in this direction,
that was never finished, and $BASE_BRANCH_GIT_COMMIT was left entirely unused,
which confused me a great deal when starting to work on this.

Incidentally, this change makes $BASE_BRANCH_GIT_COMMIT behave in the same way
as $GIT_COMMIT (which is already taken from the environment if set), which
is more consistent and less confusing.

Commit c579fc97df8ea41dda67e24047cae131b2425250 in jenkins-jobs.git
sets $BASE_BRANCH_GIT_COMMIT to the commit that the base branch was in
during the first build, that we're trying to reproduce.