refs: protect against chicken-and-egg recursion

refs: protect against chicken-and-egg recursion

In the preceding commits we have fixed recursion when creating the
reference backends due to a chicken-and-egg situation with "onbranch"
conditions. Unfortunately, this issue has existed for a while, and we
didn't really have a good mechanism to detect this recursion.

Improve the status quo by detecting the recursion when creating the main
reference store.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>