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.

     1

digraph G {

     2

	-1 -> 0

     3

	0 -> 1

     4

	0 -> 2

     5

	2 -> 3

     6

	1 -> 3

     7

}