Fall back to asciidoc6 html backend if html4 doesn't work.
This reenables building documentation on Debian sarge,
which broke after 9346a7fb3fe2.
#!/bin/sh
#
mkdir t
cd t
hg init
echo 0 > a
echo 0 > b
hg ci -A -m m -d "0 0"
hg rm a
hg cat a
sleep 1 # make sure mtime is changed
echo 1 > b
hg ci -m m -d "0 0"
echo 2 > b
hg cat -r 0 a
hg cat -r 0 b
hg cat -r 1 a
hg cat -r 1 b