mail.sh: include a "References" header for branch updates
Since the subject line of a branch update includes the new
hash, updates for the same branch name fail to thread
together if the MUA supports message threading and threading
has been enabled.
Mitigate this problem by including a "References:" header
the specifies both the branch being updated and the host
it's located on thereby allowing updates for the same
branch to be threaded together by the MUA if desired.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>