run-tests: redefine --with-hg so it takes the 'hg' script to run. - in parseargs(), check that --with-hg value is valid - add handy --local option for "--with-hg=../hg" - ensure that we always set PATH and PYTHONPATH (not just when doing a temporary install) - override any existing PYTHONPATH, so test success does not depend on whatever happens to be in the caller's environment - give tests a little more control by exporting $PYTHON to the environment; needed by test-convert and test-mergetool when they run hg with a stripped-down $PATH Also, add a big comment explaining all the corner cases to test for the next person who tries to modify this script.

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