Fall back to asciidoc6 html backend if html4 doesn't work.
This reenables building documentation on Debian sarge,
which broke after 9346a7fb3fe2.
pulling from ../B1
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg update' to get a working copy)
bar should remain deleted.
f9b0e817f6a48de3564c6b2957687c5e7297c5a0 644 foo
pulling from ../A2
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files (+1 heads)
(run 'hg update' to get a working copy)
bar should remain deleted.
f9b0e817f6a48de3564c6b2957687c5e7297c5a0 644 foo