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.
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'