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>

refs.c

diff --git a/refs.c b/refs.c
index 5b773b1..1d24637 100644
--- a/refs.c
+++ b/refs.c
@@ -2359,15 +2359,22 @@ void ref_store_release(struct ref_store *ref_store)
 
 struct ref_store *get_main_ref_store(struct repository *r)
 {
+	static bool initializing;
+
 	if (r->refs_private)
 		return r->refs_private;
 
 	if (!r->gitdir)
 		BUG("attempting to get main_ref_store outside of repository");
+	if (initializing)
+		BUG("initialization of main ref store is recursing");
 
+	initializing = true;
 	r->refs_private = ref_store_init(r, r->ref_storage_format,
 					 r->gitdir, REF_STORE_ALL_CAPS);
 	r->refs_private = maybe_debug_wrap_ref_store(r->gitdir, r->refs_private);
+	initializing = false;
+
 	return r->refs_private;
 }