git-rebase -i: mention the short command aliases in the todo list

git-rebase -i: mention the short command aliases in the todo list

git rebase -i already supports 'p', 'e' and 's' as aliases for 'pick',
'edit' and 'squash', but one could know it only by reading the source
code. If a user rebases a lot, it's quite handy, so mention these short
forms as well.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>