Add test for editing several commits right after a merge commit

This is very similar to edit_range_select_outside_rebase.go, except that it
selects commits right after, and including, a merge commit.

This test already works correctly. The reason we add it is that we are going to
have two different implementations of the `e` command depending on whether the
last selected commit is a merge commit, and we want to make sure they both work
with a range selection.

pkg/integration/tests/test_list.go

@@ -214,6 +214,7 @@ var tests = []*components.IntegrationTest{
 	interactive_rebase.EditFirstCommit,
 	interactive_rebase.EditLastCommitOfStackedBranch,
 	interactive_rebase.EditNonTodoCommitDuringRebase,
+	interactive_rebase.EditRangeSelectDownToMergeOutsideRebase,
 	interactive_rebase.EditRangeSelectOutsideRebase,
 	interactive_rebase.EditTheConflCommit,
 	interactive_rebase.FixupFirstCommit,