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
echo a > a
hg ci -Am t
hg mv a b
hg ci -Am t1
hg debugrename b
hg mv b a
hg ci -Am t2
hg debugrename a
echo % test with --rev
hg debugrename --rev 1 b