The option "--diff3" was added to "git merge-file" in e0af48e
(xdiff-merge: optionally show conflicts in "diff3 -m" style)
but it was never documented in "Documentation/git-merge-file.txt".
Add documentation for this option.
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>