setup: prefer using the system hg to interact with the local repository
Add a findhg() function that tries to be smarter about figuring out how to run
hg for examining the local repository. It first tries running "hg" from the
user's PATH, with the default HGRCPATH settings intact, but with HGPLAIN
enabled. This will generally use the same version of mercurial and the same
settings used to originally clone the repository, and should have a higher
chance of working successfully than trying to run the hg script from the local
repository. If that fails findhg() falls back to the existing behavior of
running the local hg script.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.