Adding hg version check to get_source.sh?

[Mike Duigou]

With the upgrade of the hg.openjdk.java.net servers very old mercurial clients can no longer be used (0.9.5) and there have been persistent reports of more frequent connection failures. The older your mercurial client the more likely the failures are.

                jdk:   hg clone http://hg.openjdk.java.net/jdk9/dev/jdk jdk
                jdk:   requesting all changes
                jdk:   adding changesets
                jdk:   adding manifests
                jdk:   transaction abort!
                jdk:   rollback completed
                jdk:   abort: stream ended unexpectedly (got 8372 bytes, expected 10932)

I would like to add a version check to get_source.sh which will require mercurial 1.5 or later and warn for clients older than 1.9. 

Seem reasonable?

Mike