[PATCH 0 of 3] Redefine and clarify run-tests --with-hg

[Greg Ward]

The goal of this patch series is to clarify the use of run-tests.py
--with-hg, making it more useful to developers running tests.  This is
an incompatible change: --with-hg now points to an executable 'hg'
script, not to a directory containing bin/hg.  This was discussed a
few weeks ago on this list, and no one objected loudly to the idea of
changing the meaning of --with-hg ... so here goes.