ref_update_reject_duplicates(): use `size_t` rather than `int`

Eliminate a theoretical risk of integer overflow if the two types have
different sizes.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1705,7 +1705,7 @@ int create_symref(const char *ref_target, const char *refs_heads_master,
 int ref_update_reject_duplicates(struct string_list *refnames,
 				 struct strbuf *err)
 {
-	int i, n = refnames->nr;
+	size_t i, n = refnames->nr;
 
 	assert(err);