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

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>