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.
3 files updated, 0 files merged, 0 files removed, 0 files unresolved
% merge heads
HG_FILE l
HG_MY_ISLINK 1
HG_OTHER_ISLINK 0
HG_BASE_ISLINK 0
merging l
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
% merge working directory
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
HG_FILE l2
HG_MY_ISLINK 1
HG_OTHER_ISLINK 0
HG_BASE_ISLINK 0
merging l2
0 files updated, 1 files merged, 0 files removed, 0 files unresolved