t/lib-rebase: update the documentation of FAKE_LINES

FAKE_LINES helper function use underscore to embed a space in a single
command. Let's document it and also update the list of commands.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
