Allow edit of empty message with commit --amend

"git commit --amend" used on a commit with an empty message fails
unless -m is given, whether or not --allow-empty-message is
specified.

Allow it to proceed to the editor with an empty commit message.
Unless --allow-empty-message is in force, it will still abort later
if an empty message is saved from the editor (this check was
already necessary to prevent a non-empty commit message being edited
to an empty one).

Add a test for --amend --edit of an empty commit message which fails
without this fix, as it's a rare case that won't get frequently
tested otherwise.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -690,7 +690,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		hook_arg1 = "message";
 	} else if (use_message) {
 		buffer = strstr(use_message_buffer, "\n\n");
-		if (!buffer || buffer[2] == '\0')
+		if (!use_editor && (!buffer || buffer[2] == '\0'))
 			die(_("commit has empty message"));
 		strbuf_add(&sb, buffer + 2, strlen(buffer + 2));
 		hook_arg1 = "commit";

t/t7501-commit.sh

@@ -124,6 +124,21 @@ test_expect_success '--amend --edit' '
 	test_cmp expect msg
 '
 
+test_expect_success '--amend --edit of empty message' '
+	cat >replace <<-\EOF &&
+	#!/bin/sh
+	echo "amended" >"$1"
+	EOF
+	chmod 755 replace &&
+	git commit --allow-empty --allow-empty-message -m "" &&
+	echo more bongo >file &&
+	git add file &&
+	EDITOR=./replace git commit --edit --amend &&
+	git diff-tree -s --format=%s HEAD >msg &&
+	./replace expect &&
+	test_cmp expect msg
+'
+
 test_expect_success '-m --edit' '
 	echo amended >expect &&
 	git commit --allow-empty -m buffer &&