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 bar
adding foo
% state dump
a 0 -1 baz
n 644 0 foo
r 0 0 bar
% status
A baz
R bar
C foo
% state dump
n 666 -1 bar
n 666 -1 foo
% status
! bar
? baz
C foo