Viewing a file at a specific revision should be smarter

Right now if you want to view a file at a specific point in history, it's four steps:

1. Browse to file
2. Click "History"
3. View the commit you want
4. Click "View file @ [sha]" - this may be made more difficult, or be really slow, if the commit chosen is huge

We should be able to omit step 3. It should be context-sensitive:

• If we're viewing history for a tree, there should be a link to view that tree at that revision
• If we're viewing history for a file, there should be a link to view that file at that revision