It works for submodules too.
Also, pass file name and file content explicitly; the existing tests don't care
about these, but when writing tests that do, it makes them easier to understand.
We now always re-use the state of the repo if we're returning to it, and we always reset the windows to their default tabs.
We reset to default tabs because it's easy to implement. If people want to:
* have tab states be retained when switching
* have tab states specific to the current repo retained when switching back
Then we'll need to revisit this
