tests: more completely restore the environment in syshgenv
Update the syshgenv function to attempt to completely restore the original
environment, rather than only updating a few specific variables. run_tests.py
now generates a shell script that can be used to restore the original
environment, and syshgenv sources it.
This is a bit more complicated than the previous code, but should do a better
job of running the system hg in the correct environment.
I've tested it on Linux using python 2.x, but let me know if it causes issues
in other environments. I'm not terribly familiar with how the tests get run on
Windows, for instance, and how the environment needs to be updated there.
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