Review Request: JDK-8004145: build-infra: hgforest.sh cannot be interrupted safely.

[Fredrik Öhrström]

This patch makes sure that pressing ctrl-c when running "sh 
get_source.sh" actually
sends a nice TERM signal to the background mercurial processes so that 
they can do
a nice rollback, if they need to. Really useful, when that clone has 
stalled, because
your network is down, and you want to stop and retry later.
http://cr.openjdk.java.net/~ohrstrom/webrev-8004145-hgforest/

//Fredrik