string_list: Fix argument order for string_list_insert

Update the definition and callers of string_list_insert to use the
string_list as the first argument.  This helps make the string_list
API easier to use by being more consistent.

Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/show-ref.c b/builtin/show-ref.c
index 17ada88..0b2a9ad 100644
--- a/builtin/show-ref.c
+++ b/builtin/show-ref.c
@@ -105,7 +105,7 @@
 static int add_existing(const char *refname, const unsigned char *sha1, int flag, void *cbdata)
 {
 	struct string_list *list = (struct string_list *)cbdata;
-	string_list_insert(refname, list);
+	string_list_insert(list, refname);
 	return 0;
 }