Add version check to get_source.sh

[Mike Duigou]

Hello all;

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

Mike