RFR (small): CODETOOLS-7902467: jcheck fails with mercurial 5: object of type 'branchcache' has no len()

[Tim Bell]

Mercurial release 5 removed the dict interface from the branchcache 
class.  jcheck was snooping on the length of that dict as a fast way to 
check for non default branches.

The bug report:
   https://bugs.openjdk.java.net/browse/CODETOOLS-7902467

The webrev:
   http://cr.openjdk.java.net/~tbell/CODETOOLS-7902467/webrev/

Tested on hg versions:
   2.6.2
   3.6, 3.7, 3.8, 3.9.2,
   4.5.3, 4.6.1, 4.6.2,4.7.1, 4.7.2, 4.7, 4.8.2, 4.9.1,
   5.0.1, 5.1.2, 5.2.1

Thanks in advance.

I am an Author in the Code Tools Project [1], so if this looks good, I 
would also ask a sponsor to push it for me.

Tim

[1] https://openjdk.java.net/census#code-tools