RFR: CODETOOLS-7902177: Make trees work with Mercurial 4.2 - 4.6

[Erik Joelsson]

Hello,

I recently installed Ubuntu 18.04 which comes with HG 4.5.3 and 
discovered there were issues with the trees extension again. The 
problems start with version 4.2 and up. Here is a patch that makes it 
work for all versions 1.1 to 4.6, tested with the internal test suite.

There are multiple API changes that needed to be worked around, but 
nothing here should be too controversial and I've added comments 
explaining the less obvious cases. I also had to update the URL to the 
official Mercurial repo which is used as the source for testing against 
all Mercurial versions, as well as update the tests to be compatible 
with the latest version of the Mercurial test runner and new output from 
some mercurial commands. There is also a test internal extension 
"verlist" that wasn't compatible with the 4.5.3 system mercurial on my 
new system.

Bug: https://bugs.openjdk.java.net/browse/CODETOOLS-7902177

Webrev: http://cr.openjdk.java.net/~erikj/CODETOOLS-7902177/webrev.01/

/Erik