Add test for missing commits when range-copying after a paste

After a successful paste, the "X commits copied" indicator hides and
DidPaste is set, but the buffer is not cleared. If the user then
range-selects multiple commits and presses shift+C, every Add() call
rebuilds the set via SelectedHashSet(), which returns empty while
DidPaste is true; each iteration of the copy loop therefore overwrites
the previous one and only the last commit in the range survives.