bulk-checkin: fix sign compare warnings

In file bulk-checkin.c, three warnings are emitted by
"-Wsign-compare", two of which are caused by trivial loop iterator
type mismatches.  For the third case, the type of `rsize` from

			ssize_t rsize = size < sizeof(ibuf) ? size : sizeof(ibuf);

can be changed to size_t as both options of the ternary expression are
unsigned and the signedness of the variable isn't really needed
anywhere.

To prevent `read_result != rsize` making a clash, it is to be noted
that `read_result` is checked not to hold negative values.  Therefore
casting the variable to size_t is a safe operation and enough to
remove the sign-compare warning.

Fix issues accordingly, and remove `DISABLE_SIGN_COMPARE_WARNINGS` to
enable "-Wsign-compare" for the file.

Signed-off-by: Tuomas Ahola <taahol@utu.fi>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>