hghave: add a checker for the vcr HTTP record/replay library
I'm going to use this to write some tests of the phabricator
extension.
Differential Revision: https://phab.mercurial-scm.org/D4598
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.