dashed externals: kill children on exit

dashed externals: kill children on exit

Several git commands are so-called dashed externals, that is commands
executed as a child process of the git wrapper command. If the git
wrapper is killed by a signal, the child process will continue to run.
This is different from internal commands, which always die with the git
wrapper command.

Enable the recently introduced cleanup mechanism for child processes in
order to make dashed externals act more in line with internal commands.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>