release: fix a typo in check-in-release-branch.
release: fix a typo in check-in-release-branch.

* build-aux/release.mk (check-in-release-branch): If we need to
create a new release branch, branch at the just tagged release,
rather than relying on there being no release history prior to
using this rule for the first time.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/slingshot