RFR: CODETOOLS-7901672: trees extension broken after upgrade to mercurial 3.8.1

[Erik Joelsson]

The way a Mercurial plugin interacts with Mercurial has changed over 
time. The technique used in trees has been deprecated for a while and in 
3.8, it no longer works.

I have a patch that adopts the new method using decorators, introduced 
in Mercurial 1.9. It also contains a fallback workaround that keeps 
compatibility with older versions.

For trees, there are further complications for versions between 1.9 and 
3.1 which the patch also handles. I have run the automatic tests for all 
versions from 1.1 to latest as well as manually using it with various 
versions.

Bug: https://bugs.openjdk.java.net/browse/CODETOOLS-7901672

Webrev: http://cr.openjdk.java.net/~erikj/CODETOOLS-7901672/webrev.01/

/Erik