Improve error output of git-rebase

Improve error output of git-rebase

"git rebase" without arguments on initial startup showed:

	fatal: Needed a single revision
	invalid upstream

This patch makes it show the ordinary usage string.

If .git/rebase-merge or .git/rebase-apply/rebasing exists, git-rebase
will die with a message saying that a rebase is in progress and the user
should try --skip/--abort/--continue.

If .git/rebase-apply/applying exists, git-rebase will die with a message
saying that git-am is in progress, regardless how many arguments are
given.

If no arguments are given and .git/rebase-apply/ exists, but neither a
rebasing nor applying file is in that directory, git-rebase dies with a
message saying that rebase-apply exists and no arguments were given.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>