manifest: call m1.load and m2.load before writing a subtree
As part of refactoring the manifest, certain test cases started failing because
writesubtrees was called with p1 and p2 manifests that had not been loaded (so
accessing m1._dirs resulted in an empty set). Let's call _load on these before
attempting to access _dirs.
This was caught by tests when future patches were applied.
(binary:application/octet-stream)