Clear cherry-pick buffer when copying after a paste

After a successful paste DidPaste is true, hiding the "X commits copied"
indicator but leaving the buffer populated. From the user's perspective
this looks like a clean slate, so a new shift+C should start fresh. It is
important to reset DidPaste first, before populating the buffer with the new
commits, because otherwise each loop iteration would overwrite the previous one
since Add() rebuilds the set via SelectedHashSet() which returns empty while
DidPaste is set.