hgweb: add some documentation It took longer than I wanted to grok how the various parts of hgweb worked. So I added some class and method documentation to help whoever hacks on this next.

hgweb: limit branches shown on summary page to 10 Tags and bookmarks on summary page are already limited to 10, let's limit branches as well. Each of the blocks (tags, bookmarks, branches) currently has a link to the full list.