[#4691] Temporarily skip computing diffs for removed trees pending [#5733] to reduce chance of hitting slow _diffs_copied

Signed-off-by: Cory Johns <johnsca@geek.net>
[#4691] Exclude SVN from refresh-last-commits.py since TreeDocs, TreesDocs and DiffInfoDocs are not pre-computed for SVN

Signed-off-by: Cory Johns <johnsca@geek.net>