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 8588452ceb.

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

git-commit.sh

@@ -554,7 +554,7 @@ else
 	# we need to check if there is anything to commit
 	run_status >/dev/null
 fi
-if [ "$?" != "0" -a ! -f "$GIT_DIR/MERGE_HEAD" -a -z "$amend" ]
+if [ "$?" != "0" -a ! -f "$GIT_DIR/MERGE_HEAD" ]
 then
 	rm -f "$GIT_DIR/COMMIT_EDITMSG" "$GIT_DIR/SQUASH_MSG"
 	use_status_color=t