unionrepo: don't insert index tuples with None as int field
None is not a valid size. Use -1 as placeholder instead. This will be
necessary when the index starts enforcing type correctness.
Differential Revision: https://phab.mercurial-scm.org/D9161
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.