commit: --amend -m '' silently fails to wipe message

`git commit --amend -m ''` seems to be an unambiguous request to blank a
commit message, but it actually leaves the commit message as-is.  That's
the case regardless of whether `--allow-empty-message` is specified, and
doesn't so much as drop a non-zero return code.

Add failing tests to show this behaviour.

Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7501-commit.sh

@@ -200,6 +200,26 @@ test_expect_success '--amend --edit of empty message' '
 	test_cmp expect msg
 '
 
+test_expect_failure '--amend to set message to empty' '
+	echo batá >file &&
+	git add file &&
+	git commit -m "unamended" &&
+	git commit --amend --allow-empty-message -m "" &&
+	git diff-tree -s --format=%s HEAD >msg &&
+	echo "" >expect &&
+	test_cmp expect msg
+'
+
+test_expect_failure '--amend to set empty message needs --allow-empty-message' '
+	echo conga >file &&
+	git add file &&
+	git commit -m "unamended" &&
+	test_must_fail git commit --amend -m "" &&
+	git diff-tree -s --format=%s HEAD >msg &&
+	echo "unamended" >expect &&
+	test_cmp expect msg
+'
+
 test_expect_success '-m --edit' '
 	echo amended >expect &&
 	git commit --allow-empty -m buffer &&