rebase --rebase-merges: add test for --keep-empty

If there are empty commits on the left hand side of $upstream...HEAD
then the empty commits on the right hand side that we want to keep are
being pruned.

Signed-off-by: Phillip Wood <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phillipwood

t/t3421-rebase-topology-linear.sh

@@ -217,6 +217,7 @@ test_run_rebase success ''
 test_run_rebase success -m
 test_run_rebase success -i
 test_run_rebase failure -p
+test_run_rebase success --rebase-merges
 
 #       m
 #      /