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
python -c 'print "confuse str.splitlines\nembedded\rnewline"' > a
hg ci -Ama -d '1 0'
echo clean diff >> a
hg ci -mb -d '2 0'
hg diff -r0 -r1