tests: actually restore the original environment before running syshg
Since os.environ may be overridden in run-tests.py, several important
variables such as PATH weren't restored.
I don't like the idea of using the system hg *by default* because the
executable and the configs are out of our control. But I don't mind as
long as the tests pass.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing:
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.