formatter: fix handling of None value in templater mapping
For historical reasons, None in mapping dict means there's no such keyword,
and falls back to b"". That's fine in log templates where mapping item is
generally a callable returning a value (which may be None,) but the formatter
directly puts an "evaluated" value in the mapping. So the None value has
to be lifted to wrappedvalue(None) to avoid confusion in the template engine.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.