Add core copy detection algorithm
This adds findcopies, which detects merge-relevant copies between
files in a pair of manifests back to the merge ancestor.
While the merge code invokes the copy detection routine, it does not
yet use the result.
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)