When running as root, skip the unreadable file tests, because, well,
they're probably _not_ unreadable to root unless you've got some
crazy NSA clearance-level honoring operating system shit going on.
Allow callers to specify a start path with a trailing slash to match
a submodule, instead of just a directory. This is for some legacy
behavior that's sort of dumb, but there it is.
Iterator tests were split over repo::iterator and diff::iterator,
with duplication between the two. Move them to iterator::index,
iterator::tree, and iterator::workdir.
