Make it possible to disable filtering for a pattern.
Rationale: Apparently there were no way to disable for example win32text extension locally.
RFC: Can it already be done another way?
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "hgk=" >> $HGRCPATH
hg init repo
cd repo
echo a > a
hg ci -Am adda
hg debug-cat-file commit 0