RFR: Make trees work with hg 4.6

[Erik Joelsson]

Hello,

I've rebased my patch for getting trees to work with hg 4.6 on top of 
Mark's latest commits. This patch also fixes an issue with the internal 
verlist plugin if the default mercurial on the system running the tests 
is newer than 3.8. It also widens the filtering of release candidate 
tags in the hg main repo to catch the 4.6rc1 and 4.6rc2 tags.

Webrev: http://cr.openjdk.java.net/~erikj/trees-4.6/webrev.01/

/Erik