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.
% create source repository
adding a
% fork source repository
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
% update source repository
% merge repositories
pulling from ../t2
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
% rename b as c
A c
R b
% rename back c as b