merge: avoid a negation in the definition of updatedirstate
We only use `partial` in one place: the definition of
`updatedirstate`. Let's simplify that a little.
Differential Revision: https://phab.mercurial-scm.org/D7900
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.