mailmap: change primary address for Emily Shaffer

Emily finally figured out how to set up their alias at DayJob, and would
prefer to use nasamuffin@google.com, partially to reduce confusion
between IRC and list, and partially because they just like the alias a
lot more.

Signed-off-by: Emily Shaffer <nasamuffin@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.mailmap

@@ -65,6 +65,7 @@ Derrick Stolee <derrickstolee@github.com> <dstolee@microsoft.com>
 Deskin Miller <deskinm@umich.edu>
 Đoàn Trần Công Danh <congdanhqx@gmail.com> Doan Tran Cong Danh
 Dirk Süsserott <newsletter@dirk.my1.cc>
+Emily Shaffer <nasamuffin@google.com> <emilyshaffer@google.com>
 Eric Blake <eblake@redhat.com> <ebb9@byu.net>
 Eric Hanchrow <eric.hanchrow@gmail.com> <offby1@blarg.net>
 Eric S. Raymond <esr@thyrsus.com>