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
#require black $ cd $RUNTESTDIR/.. $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`