rhg: Add environment variables for fallback configuration...
rhg: Add environment variables for fallback configuration
For the `rust-tests.py --rhg` we want every `hg` command in tests
to run `rhg` with fallback enabled, but other methods of setting
configuration are limited or disruptive as discussed in code comment.
Differential Revision:
https://phab.mercurial-scm.org/D10186
