Merge branch 'cw/amend-commit-without-message' into maint

"commit --amend" used to refuse amending a commit with an empty log
message, with or without "--allow-empty-message".

* cw/amend-commit-without-message:
  Allow edit of empty message with commit --amend

builtin/commit.c

@@ -643,7 +643,7 @@ 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

@@ -148,6 +148,21 @@ 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 &&