copies._forwardcopies: use set operations to find missing files This is a performance win for a number of reasons: - We don't iterate over contexts, which avoids a completely unnecessary sorted call + the O(number of files) abstraction cost of doing that. - We don't check membership in a context, which avoids another O(number of files) abstraction cost. - We iterate over the manifests in C instead of Python. For a large repo with 170,000 files, this improves perfpathcopies from 0.34 seconds to 0.07. Anything that uses pathcopies, such as rebase or diff --git between two revisions, benefits.