invalidate_ref_cache(): expose this function in the refs API

Make invalidate_ref_cache() an official part of the refs API.  It is
currently a fact of life that code outside of refs.c mucks about with
references.  This change gives such code a way of informing the refs
module that it should no longer trust its cache.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs.c

@@ -202,7 +202,7 @@ static struct ref_cache *get_ref_cache(const char *submodule)
 	return refs;
 }
 
-static void invalidate_ref_cache(const char *submodule)
+void invalidate_ref_cache(const char *submodule)
 {
 	clear_ref_cache(get_ref_cache(submodule));
 }

refs.h

@@ -80,6 +80,14 @@ extern void unlock_ref(struct ref_lock *lock);
 /** Writes sha1 into the ref specified by the lock. **/
 extern int write_ref_sha1(struct ref_lock *lock, const unsigned char *sha1, const char *msg);
 
+/*
+ * Invalidate the reference cache for the specified submodule.  Use
+ * submodule=NULL to invalidate the cache for the main module.  This
+ * function must be called if references are changed via a mechanism
+ * other than the refs API.
+ */
+extern void invalidate_ref_cache(const char *submodule);
+
 /** Setup reflog before using. **/
 int log_ref_setup(const char *ref_name, char *logfile, int bufsize);