refs.h: let for_each_namespaced_ref() take excluded patterns

A future commit will want to call `for_each_namespaced_ref()` with
a list of excluded patterns.

We could introduce a variant of that function, say,
`for_each_namespaced_ref_exclude()` which takes the extra parameter, and
reimplement the original function in terms of that. But all but one
caller (in `http-backend.c`) will supply the new parameter, so add the
new parameter to `for_each_namespaced_ref()` itself instead of
introducing a new function.

For now, supply NULL for the list of excluded patterns at all callers to
avoid changing behavior, which we will do in a future change.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -376,7 +376,12 @@ int for_each_glob_ref_in(each_ref_fn fn, const char *pattern,
 			 const char *prefix, void *cb_data);
 
 int head_ref_namespaced(each_ref_fn fn, void *cb_data);
-int for_each_namespaced_ref(each_ref_fn fn, void *cb_data);
+/*
+ * references matching any pattern in "exclude_patterns" are omitted from the
+ * result set on a best-effort basis.
+ */
+int for_each_namespaced_ref(const char **exclude_patterns,
+			    each_ref_fn fn, void *cb_data);
 
 /* can be used to learn about broken ref and symref */
 int refs_for_each_rawref(struct ref_store *refs, each_ref_fn fn, void *cb_data);