patch 8.1.1746: ":dl" is seen as ":dlist" instead of ":delete"

Problem:    ":dl" is seen as ":dlist" instead of ":delete".
Solution:   Do not use cmdidxs2[] if the length is 1. (closes #4721)

src/testdir/test_excmd.vim

@@ -0,0 +1,10 @@
+" Tests for various Ex commands.
+
+func Test_ex_delete()
+  new
+  call setline(1, ['a', 'b', 'c'])
+  2
+  " :dl is :delete with the "l" flag, not :dlist
+  .dl
+  call assert_equal(['a', 'c'], getline(1, 2))
+endfunc