git-rebase--preserve-merges: fix formatting of todo help message

git-rebase--preserve-merges: fix formatting of todo help message

Part of the todo help message in git-rebase--preserve-merges.sh is
unnecessarily indented, making the message look weird.  Remove the
extra lines and trailing indent.

This was a minor regression introduced by d48f97aa ("rebase:
reindent function git_rebase__interactive", 2018-03-23) in the 2.18
timeframe.  The same issue exists in "rebase -i", but it is being
addressed separately as part of the rewrite of the subcommand into C.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>