Check for parent/mod as well as parent.mod in findext. Debian/Ubuntu ship a system-wide hgrc that loads extensions using the syntax hgext/extension=, rather than hgext.extension=. Although the extensions work correctly, hg help <extension> doesn't understand this form without this patch.

#!/bin/sh
#
mkdir t
cd t
hg init
echo 0 > a
echo 0 > b
hg ci -A -m m -d "1000000 0"
hg rm a
hg cat a
sleep 1 # make sure mtime is changed
echo 1 > b
hg ci -m m -d "1000000 0"
echo 2 > b
hg cat -r 0 a
hg cat -r 0 b
hg cat -r 1 a
hg cat -r 1 b