Allow the branch selector in the source browser to accept arbitrary commits (BB-7005)

Even though there is still no way to go directly from a commit to the source from that commit, now the 'Source' tab is completely ruined as well. It is now impossible to list the sources of a previous commit. Instead of the useless 'raw commit' button, which I doubt anyone uses, please create a way to go directly from commit to source AND give us the option back to browse the source from a specific commit. You now have to browse manually to the source and paste the commit id in the url, which is just rubbish. And when you are at it, please also allow that version to be downloaded through zip file or something.