[#5781] Changed prev/next links to not auto-vivify LCDs or Trees

Signed-off-by: Cory Johns <johnsca@geek.net>
[#5781] Added test coverage for Commit.get_path, Commit.get_tree, and Blob.prev/next_commit

Signed-off-by: Cory Johns <johnsca@geek.net>
[#5781] Fixed failing LastCommit tests

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