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.
#!/bin/sh
echo '[Section]' >> $HGRCPATH
echo 'KeY = Case Sensitive' >> $HGRCPATH
echo 'key = lower case' >> $HGRCPATH
hg showconfig Section