context: use includematcher when checking dir/file conflicts

Authored by spectral.

Description

context: use includematcher when checking dir/file conflicts

This is for performance; patternmatcher can't easily interpret its results to
make visitchildrenset be the "optimal" set of paths to inspect, but
includematcher can. Since there aren't any special patterns being used here, I
believe that the two matchers are equivalent.

Differential Revision: https://phab.mercurial-scm.org/D5999

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