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

git-rebase--interactive.sh

@@ -530,9 +530,9 @@ do
 # Rebase $SHORTUPSTREAM..$SHORTHEAD onto $SHORTONTO
 #
 # Commands:
-#  pick = use commit
-#  edit = use commit, but stop for amending
-#  squash = use commit, but meld into previous commit
+#  p, pick = use commit
+#  e, edit = use commit, but stop for amending
+#  s, squash = use commit, but meld into previous commit
 #
 # If you remove a line here THAT COMMIT WILL BE LOST.
 # However, if you remove everything, the rebase will be aborted.