merge-tree: improve docs for --stdin

merge-tree: improve docs for --stdin

Add a section for --stdin in the list of options and document that it
implies -z so readers know how to parse the output. Also correct the
merge status documentation for --stdin as if the status is less than
zero "git merge-tree" dies before printing it.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>