refactor find_ref_by_name() to accept const list

Since it doesn't actually touch its argument, this makes
sense.

However, we still want to return a non-const version (which
requires a cast) so that this:

  struct ref *a, *b;
  a = find_ref_by_name(b);

works. Unfortunately, you can also silently strip the const
from a variable:

  struct ref *a;
  const struct ref *b;
  a = find_ref_by_name(b);

This is a classic C const problem because there is no way to
say "return the type with the same constness that was passed
to us"; we provide the same semantics as standard library
functions like strchr.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -801,7 +801,7 @@ struct ref {
 #define REF_HEADS	(1u << 1)
 #define REF_TAGS	(1u << 2)
 
-extern struct ref *find_ref_by_name(struct ref *list, const char *name);
+extern struct ref *find_ref_by_name(const struct ref *list, const char *name);
 
 #define CONNECT_VERBOSE       (1u << 0)
 extern struct child_process *git_connect(int fd[2], const char *url, const char *prog, int flags);

refs.c

@@ -1628,10 +1628,10 @@ int update_ref(const char *action, const char *refname,
 	return 0;
 }
 
-struct ref *find_ref_by_name(struct ref *list, const char *name)
+struct ref *find_ref_by_name(const struct ref *list, const char *name)
 {
 	for ( ; list; list = list->next)
 		if (!strcmp(list->name, name))
-			return list;
+			return (struct ref *)list;
 	return NULL;
 }