ftdetect/git.vim: Fix gitsendemail detection

In modern versions of git send-email, messages are named
.gitsendemail.msg.([a-zA-Z0-9]|_)* (I haven't read the code to be sure
there aren't more options for the unique identifier), but currently the
file detection assumes that they are named .msg.[0-9]*, obviously this
regex will not match. The easiest solution is just to look for
.gitsendemail.*

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>