.mailmap: update email address for Tian Yuchen

Map my old Gmail address to my new custom address in .mailmap.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: malon7782

.mailmap

@@ -283,6 +283,7 @@ Thomas Ackermann <th.acker@arcor.de> <th.acker66@arcor.de>
 Thomas Rast <tr@thomasrast.ch> <trast@student.ethz.ch>
 Thomas Rast <tr@thomasrast.ch> <trast@inf.ethz.ch>
 Thomas Rast <tr@thomasrast.ch> <trast@google.com>
+Tian Yuchen <cat@malon.dev> <a3205153416@gmail.com>
 Timo Hirvonen <tihirvon@gmail.com> <tihirvon@ee.oulu.fi>
 Toby Allsopp <Toby.Allsopp@navman.co.nz> <toby.allsopp@navman.co.nz>
 Tom Grennan <tmgrennan@gmail.com> <tgrennan@redback.com>