convert: save hg revision during a convert
This breaks hg->hg hash roundtripping, but that was fragile, didn't work
reliably, and hence arguably of little value.
#!/bin/sh
hg init
python -c 'print "confuse str.splitlines\nembedded\rnewline"' > a
hg ci -Ama -d '1 0'
echo clean diff >> a
hg ci -mb -d '2 0'
hg diff -r0 -r1