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.
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t