patch 8.0.1242: function argument with only dash is seen as number zero

Problem:    Function argument with only dash is seen as number zero. (Wang
            Shidong)
Solution:   See a dash as a string. (Christian Brabandt)
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index bdad26a..d25267f 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -90,3 +90,22 @@
   call delete('Xtestdata')
   set cpt& cot& def& tags& tagbsearch& hidden&
 endfunc
+
+func Test_omni_dash()
+  func Omni(findstart, base)
+    if a:findstart
+        return 5
+    else
+        echom a:base
+	return ['-help', '-v']
+    endif
+  endfunc
+  set omnifunc=Omni
+  new
+  exe "normal Gofind -\<C-x>\<C-o>"
+  call assert_equal("\n-\nmatch 1 of 2", execute(':2mess'))
+
+  bwipe!
+  delfunc Omni
+  set omnifunc=
+endfunc