If the user runs the blame viewer on a working directory file
instead of a specific commit-ish then we have no value for the
commit SHA1 or the summary line; this causes the history menu
to get an empty entry at the very bottom. We now look for this
odd case and call the meny entry "Working Directory".
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>