git-commit: Disallow amend if it is going to produce an empty non-merge commit

git-commit: Disallow amend if it is going to produce an empty non-merge commit

Right now one can amend the last non-merge commit using a dirty index
and in the process maybe cause the last commit to have the same tree
as its parent.  In such a case one would want to discard the last commit
instead of amending it.

This reverts commit 8588452ceb78b1da17652ba03f9942ef740e07ea.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>