global: improve const correctness when assigning string constants

We're about to enable `-Wwrite-strings`, which changes the type of
string constants to `const char[]`. Fix various sites where we assign
such constants to non-const variables.

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

refs.h

@@ -975,7 +975,7 @@ struct ref_store *get_worktree_ref_store(const struct worktree *wt);
  */
 
 struct ref_namespace_info {
-	char *ref;
+	const char *ref;
 	enum decoration_type decoration;
 
 	/*