tests: add test for narrow+share
For how narrow+share is supposed to work, see
https://www.mercurial-scm.org/wiki/NarrowSharePlan.
Differential Revision: https://phab.mercurial-scm.org/D5276
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.