beautifygraph: don't substitute anything for 'X' in rendered graphs
It looks like we never actually render graphs this way, although there's an
example in a comment next to the code that prevents it from being needed (see
graphmod.ascii()).
Differential Revision: https://phab.mercurial-scm.org/D5103
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.