rebase usage: subcommands can not be combined with -i

Since 95135b0 (rebase: stricter check of standalone sub command,
2011-02-06), git-rebase has not allowed to use -i together with e.g.
--continue. Yet, when rebase started using OPTIONS_SPEC in 45e2acf
(rebase: define options in OPTIONS_SPEC, 2011-02-28), the usage message
included

	git-rebase [-i] --continue | --abort | --skip

Remove the "[-i]" from this line.

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -8,7 +8,7 @@ OPTIONS_KEEPDASHDASH=
 OPTIONS_SPEC="\
 git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] [<upstream>] [<branch>]
 git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] --root [<branch>]
-git-rebase [-i] --continue | --abort | --skip
+git-rebase --continue | --abort | --skip
 --
  Available options are
 v,verbose!         display a diffstat of what changed upstream