git-commit: allow --only to lose what was staged earlier.

git-commit: allow --only to lose what was staged earlier.

The command used to have a safety valve to prevent this sequence:

	edit foo
	git update-index foo
	edit foo
	git diff foo
	git commit --only foo

The reason for this was because an inexperienced user might
mistakenly think what is shown with the last-minute diff
contains all the change that is being committed (instead, what
the user asked to check was an incremental diff since what has
been staged so far).  However, this turns out to only annoy
people who know what they are doing.  Inexperienced people
would not be using the first "update-index" anyway, in which
case they would see the full changes in the "git diff".

Signed-off-by: Junio C Hamano <junkio@cox.net>