diff --check: detect leftover conflict markers

diff --check: detect leftover conflict markers

This teaches "diff --check" to detect and complain if the change
adds lines that look like leftover conflict markers.

We should be able to remove the old Perl script used in the sample
pre-commit hook and modernize the script with this facility.

Signed-off-by: Junio C Hamano <gitster@pobox.com>