[Bug-gnubg] Save as Position discards existing analysis

[M. J. Mannon]

I recently received a rollout file which opened as a “match” of unlimited 
games. Since I thought it would be nice to strip away the surrounding match 
information, and isolate just the rollout of one move, I tried selecting the 
move, and then saving it as a “position.” Imagine my surprise to discover that 
the rollout was discarded along with the other moves in the match.
 
It would be reasonable for Save as Position to save any analysis that already 
exists when this commmand is chosen.