Add file path to browse command tree subpage

Hi,

Many times, I would want to show highlighted code to other people. The GitHub interface for traversing through files is slow. Though I can type the file path in the url myself, I miss the autocompletion for files. This pull request will allow:

`git browse github/hub tree lib/hub.rb#L1-8` and it'll open the url `https://github.com/github/hub/tree/master/lib/hub.rb#L1-8`

Let me know what you think! (And if I need to update any documentation)