narrow: pass 'narrow_widen' as source while generating changegroup
Extensions inspect the `source` parameter and add some custom logic on the basis
of value of that parameter. The parameter is also passed to hooks. So let's pass
the value as 'narrow_widen' to differentiate widening from pull.
Differential Revision: https://phab.mercurial-scm.org/D4787
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh