Set \origin correctly when upgrading docs

Set \origin correctly when upgrading docs

lyx2lyx did not yet know about /systemlyxdir/ and set \origin to the path
where my git tree lives instead. This path is not usable except on my machine,
so better write something more usable instead.
This is a special command line switch of lyx2lyx, so it does not interfere
with normal usage. I did not try to deduce the systemlyxdir from lyx2lyx to
be on the safe side.