run-tests: do not inherit file descriptor when running a command
This is one of the difference between python2 and python3 and could have been a
reason why test hang with python2 + chg. This does not seems to help the
hanging issue at all…
However, now that this is written lets reduce the difference between python2
and python3.
Differential Revision: https://phab.mercurial-scm.org/D11226
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