ftdetect/git.vim: Fix gitsendemail detection

In modern versions of git send-email (Since Nov 2008), messages are
named .gitsendemail.msg.<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.*

v2: - simplify the check for gitsendemail

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>