Honor $BASE_BRANCH_GIT_COMMIT (refs: #16730)
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.
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