git-send-email: SIG{TERM,INT} handlers

git-send-email: SIG{TERM,INT} handlers

A single signal handler is used for both SIGTERM and
SIGINT in order to clean up after an uncouth termination
of git-send-email.

In particular, the handler resets the text color (this cleanup
was already present), turns on tty echoing (in case termination
occurrs during a masked Password prompt), and informs the user
of of any temporary files created by --compose.

Signed-off-by: Michael Witten <mfwitten@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>