builtin/repack.c: avoid unnecessary numeric casts in existing_packs

There are a couple of spots that cause warnings within the
existing_packs API without DISABLE_SIGN_COMPARE_WARNINGS under
DEVELOPER=1 mode.

In both cases, we have int values that are being compared against size_t
ones. Neither of these two cases are incorrect, and the cast is
completely OK in practice. But both are unnecessary, since:

 - in existing_packs_mark_for_deletion_1(), 'hexsz' should be defined as
   a size_t anyway, since algop->hexsz is.

 - in existing_packs_collect(), 'i' should be defined as a size_t since
   it is counting up to the value of a string_list's 'nr' field.

(This patch is a little bit of noise, but I would rather see us squelch
these warnings ahead of moving the existing_packs API into a separate
compilation unit to avoid having to define DISABLE_SIGN_COMPARE_WARNINGS
in repack.c.)

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

Author: ttaylorr

builtin/repack.c

@@ -156,7 +156,7 @@ static void existing_packs_mark_for_deletion_1(const struct git_hash_algo *algop
 					       struct string_list *list)
 {
 	struct string_list_item *item;
-	const int hexsz = algop->hexsz;
+	const size_t hexsz = algop->hexsz;
 
 	for_each_string_list_item(item, list) {
 		char *sha1;
@@ -250,7 +250,7 @@ static void existing_packs_collect(struct existing_packs *existing,
 	struct strbuf buf = STRBUF_INIT;
 
 	for (p = packfile_store_get_all_packs(packs); p; p = p->next) {
-		int i;
+		size_t i;
 		const char *base;
 
 		if (!p->pack_local)