Clear RBUFFER when accepting output from atuin (#545)

Since we pass $BUFFER to atuin search, retaining RBUFFER (the part of
the buffer to the right of the cursor) probably doesn't make sense. The
advantage of setting RBUFFER and LBUFFER separately instead of setting
BUFFER is that the cursor is positioned after the end of LBUFFER instead
of remaining where it was before atuin was called.

src/shell/atuin.zsh

@@ -38,6 +38,7 @@ _atuin_search(){
 	echoti smkx
 
 	if [[ -n $output ]] ; then
+		RBUFFER=""
 		LBUFFER=$output
 	fi