D5532: context: schedule file prefetch before comparing for cleanliness

[Yuya Nishihara]

> +        scmutil.prefetchfiles(
> +            self.repo(), [self.p1().rev()],
> +            matchmod.match('', '', patterns=self._cache.keys(), exact=True))

Perhaps `scmutil.matchfiles()` can be used instead of the low-level constructor.