reencode_string: use size_t for string lengths

reencode_string: use size_t for string lengths

The iconv interface takes a size_t, which is the appropriate
type for an in-memory buffer. But our reencode_string_*
functions use integers, meaning we may get confusing results
when the sizes exceed INT_MAX. Let's use size_t
consistently.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>