http-backend: enable cleaning up forked upload/receive-pack on exit

http-backend: enable cleaning up forked upload/receive-pack on exit

If http-backend dies because of errors, started upload-pack or
receive-pack are not killed and waited, but rather stay running for
some time until they exit because of closed stdin. It may be
undesirable in working environment, and it also causes occasional
failure of t5562, because the processes keep opened act.err, and
sometimes write there errors after next test started using the file.

Fix by enabling cleaning of the command at http-backed exit.

Reported-by: Carlo Arenas <carenas@gmail.com>
Helped-by: Carlo Arenas <carenas@gmail.com>
Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>