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

hg init a
cd a
hg init b
echo x > b/x
echo '# should print nothing'
hg st
echo '# should print ? b/x'
hg st b/x

hg add b/x

echo '# should print A b/x'
hg st
echo '# should forget b/x'
hg revert
echo '# should print nothing'
hg st b