git-commit --amend: two fixes.

git-commit --amend: two fixes.

When running "git commit --amend" only to fix the commit log
message without any content change, we mistakenly showed the
git-status output that says "nothing to commit" without
commenting it out.

If you have already run update-index but you want to amend the
top commit, "git commit --amend --only" without any paths should
have worked, because --only means "starting from the base
commit, update-index these paths only to prepare the index to
commit, and perform the commit".  However, we refused -o without
paths.

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