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
hg init test
cd test
mkdir da
echo foo > da/foo
echo foo > foo
hg ci -Ambase -d '0 0'
hg serve -p $HGPORT -d --pid-file=hg.pid
echo % manifest
("$TESTDIR/get-with-headers.py" localhost:$HGPORT '/file/tip/?style=raw')
("$TESTDIR/get-with-headers.py" localhost:$HGPORT '/file/tip/da?style=raw')
kill `cat hg.pid`