report: show proper info for merging origin

report: show proper info for merging origin

When there is 'merging' of an origin in progress, but metadata stil
do provide both origin and snapshot, we should show data from merged
snapshot.  This is important mainly for thin case, where there was
a window, where i.e. 'lvs -o+device_id' would report information
about 'already gone' origin thin LV.

This race window is usually hard to trigger but can be ocasionally hit.
Usually shortly after activation, but before polling process manages
to update metadata after merge.