Use strchrnul() instead of strchr() plus manual workaround

Also gets rid of a C++ comment.

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

builtin-for-each-ref.c

@@ -320,9 +320,7 @@ 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) // simulate strchrnul()
-		eol = buf + strlen(buf);
+	const char *eol = strchrnul(buf, '\n');
 	return xmemdupz(buf, eol - buf);
 }