cat: default to working dir parent instead of tip
This introduces a defaultrev function that chooses
the working dir parent if a revision isn't specified,
and uses it in several places.
Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory