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.

#!/bin/sh

hg
hg -q
hg help
hg -q help
hg add -h
hg add --skjdfks
hg help diff
hg help status
hg -q help status
hg help foo
hg skjdfks

exit 0