environment: stop storing "core.notesRef" globally

Stop storing the "core.notesRef" config value globally. Instead,
retrieve the value in `default_notes_ref()`. The code is never called in
a hot loop anyway, so doing this on every invocation should be perfectly
fine.

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

notes.h

@@ -4,6 +4,7 @@
 #include "string-list.h"
 
 struct object_id;
+struct repository;
 struct strbuf;
 
 /*
@@ -70,7 +71,7 @@ extern struct notes_tree {
  * 3. The value of the core.notesRef config variable, if set
  * 4. GIT_NOTES_DEFAULT_REF (i.e. "refs/notes/commits")
  */
-const char *default_notes_ref(void);
+char *default_notes_ref(struct repository *repo);
 
 /*
  * Flags controlling behaviour of notes tree initialization