git-commit: populate the edit buffer with 2 blank lines before s-o-b

'commit -s' populates the edit buffer with a blank line before the
Signed-off-by line, to allow the user to immediately start typing
the log message.  But commit 33f2f9ab removed this space, forcing
the user to first push the Signed-off-by line down to open a place
to type the log message.

Fix this regression and let's ensure that the Signed-off-by line is
preceded by two blank lines, instead of just one, to hint that
something should be filled in, and that a blank line should separate
it from the body and the Signed-off-by line.

Add a test for this behavior.

Reported-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -1127,10 +1127,33 @@ void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag)
 		const char *append_newlines = NULL;
 		size_t len = msgbuf->len - ignore_footer;
 
-		if (len && msgbuf->buf[len - 1] != '\n')
+		if (!len) {
+			/*
+			 * The buffer is completely empty.  Leave foom for
+			 * the title and body to be filled in by the user.
+			 */
 			append_newlines = "\n\n";
-		else if (len > 1 && msgbuf->buf[len - 2] != '\n')
+		} else if (msgbuf->buf[len - 1] != '\n') {
+			/*
+			 * Incomplete line.  Complete the line and add a
+			 * blank one so that there is an empty line between
+			 * the message body and the sob.
+			 */
+			append_newlines = "\n\n";
+		} else if (len == 1) {
+			/*
+			 * Buffer contains a single newline.  Add another
+			 * so that we leave room for the title and body.
+			 */
 			append_newlines = "\n";
+		} else if (msgbuf->buf[len - 2] != '\n') {
+			/*
+			 * Buffer ends with a single newline.  Add another
+			 * so that there is an empty line between the message
+			 * body and the sob.
+			 */
+			append_newlines = "\n";
+		} /* else, the buffer already ends with two newlines. */
 
 		if (append_newlines)
 			strbuf_splice(msgbuf, msgbuf->len - ignore_footer, 0,

t/t7502-commit.sh

@@ -349,6 +349,18 @@ test_expect_success 'A single-liner subject with a token plus colon is not a foo
 
 '
 
+test_expect_success 'commit -s places sob on third line after two empty lines' '
+	git commit -s --allow-empty --allow-empty-message &&
+	cat <<-EOF >expect &&
+
+
+	Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>
+
+	EOF
+	sed -e "/^#/d" -e "s/^:.*//" .git/COMMIT_EDITMSG >actual &&
+	test_cmp expect actual
+'
+
 write_script .git/FAKE_EDITOR <<\EOF
 mv "$1" "$1.orig"
 (