diff.h: fix index used to loop through unsigned integer

diff.h: fix index used to loop through unsigned integer

The `struct diff_flags` structure is essentially an array of flags, all
of which have the same type. We can thus use `sizeof()` to iterate
through all of the flags, which we do in `diff_flags_or()`. But while
the statement returns an unsigned integer, we used a signed integer to
iterate through the flags, which generates a warning.

Fix this by using `size_t` for the index instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>