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>

git-rebase--preserve-merges.sh

@@ -891,9 +891,9 @@ $comment_char $(eval_ngettext \
 EOF
 	append_todo_help
 	gettext "
-	However, if you remove everything, the rebase will be aborted.
+However, if you remove everything, the rebase will be aborted.
 
-	" | git stripspace --comment-lines >>"$todo"
+" | git stripspace --comment-lines >>"$todo"
 
 	if test -z "$keep_empty"
 	then