rhg: Add support for --cwd...
rhg: Add support for --cwd
This affect the meaning of relative paths in `--repository`,
which are resolved "early" by rhg in order to load config
which is needed before fallback to Python is considered.
An incorrect path could cause errors when loading a non-existent repo,
leading to failing tests even when fallback is enabled.
Differential Revision:
https://phab.mercurial-scm.org/D10134