diff: drop useless "status" parameter from diff_result_code()

diff: drop useless "status" parameter from diff_result_code()

Many programs use diff_result_code() to get a user-visible program exit
code from a diff result (e.g., checking opts.found_changes if
--exit-code was requested).

This function also takes a "status" parameter, which seems at first
glance that it could be used to propagate an error encountered when
computing the diff. But it doesn't work that way:

  - negative values are passed through as-is, but are not appropriate as
    program exit codes

  - when --exit-code or --check is in effect, we _ignore_ the passed-in
    status completely. So a failed diff which did not have a chance to
    set opts.found_changes would erroneously report "success, no
    changes" instead of propagating the error.

After recent cleanups, neither of these bugs is possible to trigger, as
every caller just passes in "0". So rather than fixing them, we can
simply drop the useless parameter instead.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>