tests/helper-runtests.sh

templater: add inheritance support to style maps

We can now specify a base map file:

__base__ = path/to/map/file

That map file will be read and used to populate unset elements of the
current map. Unlike using %include, elements in the inherited class
will be read relative to that path.

This makes it much easier to make custom local tweaks to a style.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL