pre-commit hook: complain about conflict markers.

Several <<< or === or >>> characters at the beginning of a line
is very likely to be leftover conflict markers from a failed
automerge the user resolved incorrectly, so detect them.

As usual, this can be defeated with "git commit --no-verify" if
you really do want to have those files, just like changes that
introduce trailing whitespaces.

Signed-off-by: Junio C Hamano <junkio@cox.net>