Add version check to get_source.sh

[Tim Bell]

Hi Mike:

> This is the changeset for the enhancement proposed last week. It adds version checks to get_source.sh script. Currently the script will abort if the Mercurial client is not found or is older than 1.5.0. If Mercurial client is older than 2.6.3 then a warning will be issued but the script will attempt to continue.
>
> webrev: http://cr.openjdk.java.net/~mduigou/JDK-8047925/0/webrev/
> jbsbug: https://bugs.openjdk.java.net/browse/JDK-8047925

Looks good to me.

Tim