for-each-ref: Fix --format=%(subject) for log message without newlines

'git for-each-ref --format=%(subject)' currently returns an empty string
if the log message does not contain a newline.

This patch teaches 'git for-each-ref' to return the entire log message
(instead of an empty string) if there is no newline in the log message.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-for-each-ref.c

@@ -321,8 +321,8 @@ static const char *find_wholine(const char *who, int wholen, const char *buf, un
 static const char *copy_line(const char *buf)
 {
 	const char *eol = strchr(buf, '\n');
-	if (!eol)
-		return "";
+	if (!eol) // simulate strchrnul()
+		eol = buf + strlen(buf);
 	return xmemdupz(buf, eol - buf);
 }