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.

adding da/foo
adding foo
% manifest
200 Script output follows


drwxr-xr-x da
-rw-r--r-- 4 foo


200 Script output follows


-rw-r--r-- 4 foo