A new mercurial loader. It works directly on version 2 bundle files, which is (I believe) the current mercurial transport format. This is much faster than bouncing through hglib for everything. There are a few bits and variables to quantify. See especially any TODO and NOTE comments, of course.
No rush on the review. It's a lot of code all at once. ^o^
After review, will need to switch DEBUG in bundle20_loader.py from True to False.
No official unit tests yet, but there is validation code (bundle20_loader_verifier.py) for contents and directories comparing against hglib output.