convert: make contents of "extra" dict available from sources, for sinks.
This breaks hash preservation for hg->hg conversion, as each converted
change gets a convert_revision item added to its extra dict. Ugh.
#!/bin/sh
#
hg init t
cd t
echo 1 > foo
hg ci -Am m
cd ..
hg clone t tt
cd tt
echo 1.1 > foo
hg ci -Am m
cd ../t
echo 1.2 > foo
hg ci -Am m
echo % should fail
hg pull -u ../tt
cd ../tt
echo % should fail
hg pull -u ../t
HGMERGE=true hg merge
hg ci -mm
cd ../t
echo % should work
hg pull -u ../tt