Add the possibility to browse other revisions

[mbonaker]

The Idea would be, that one could select the current revision and then the file browser would behave as if that revision was checked out.

The selection of the revision could maybe work like this:
Show a list all references (tags, branches and maybe stashes) with 2 buttons:

1. View the HEAD of that reference
2. Show the log of that reference
   And additionally all logs (i.e. every appearance of a revision) should have an option to view it.

This should be possible in theory because we already are in power of the storage driver for the current list view...

[mbonaker]

For bare repositories it should not be possible to view the raw files but instead the user should be offered the choice, which revision they want to view.