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 init a
echo a > a/a
hg --cwd a ci -A -m a
hg clone a b
echo b > b/b
hg --cwd b ci -A -m b
chmod 100 a/.hg
hg --cwd b push ../a
chmod 700 a/.hg