This makes it possible to run e.g. "yadm -Y foo introspect repo
2>/dev/null" and get an empty output instead of getting the error
message about foo not being fully qualified.
* Create a YADM_DATA base (that is the source for repo & archive)
* Add --yadm-data to override
* Default YADM_DATA will be determined by XDG_DATA_HOME
