* Make sure we are in the top level working directory. This
way we can access files using their repository path.
* Reload the diff viewer if the current file's status has changed;
as the diff may now be different.
* Correctly handle the 'AD' file state: added but now gone from
the working directory.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>