common-main: call restore_sigpipe_to_default()

common-main: call restore_sigpipe_to_default()

This is another safety/sanity setup that should be in force
everywhere, but which we only applied in git.c. This did
catch most cases, since even external commands are typically
run via "git ..." (and the restoration applies to
sub-processes, too). But there were cases we missed, such as
somebody calling git-upload-pack directly via ssh, or
scripts which use dashed external commands directly.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>