RFR: 8074910 hgforest.sh needs an option to bring over a smaller set of extra repos

[Phil Race]

Right now to get the extra repos you do
sh get_source.sh http://server.whatits.org

This gets more than most people need which wastes time and disk.
Here is a small change such that you can now do

sh get_source.sh jdk http://server.whatits.org

ie add "jdk" to indicate you'd like only the repos needed to build JDK.

If you decide later you want them all, just repeat using the old form.

If you are unaware of the new arg, nothing changes.

https://bugs.openjdk.java.net/browse/JDK-8074910
http://cr.openjdk.java.net/~prr/8074910/

-phil.