patch 7.4.2219

Problem:    Recursive call to substitute gets stuck in sandbox. (Nikolai
            Pavlov)
Solution:   Handle the recursive call. (Christian Brabandt, closes #950)
            Add a test.

src/testdir/test_regexp_latin.vim

@@ -30,3 +30,12 @@ func Test_equivalence_re2()
   set re=2
   call s:equivalence_test()
 endfunc
+
+func Test_recursive_substitute()
+  new
+  s/^/\=execute("s#^##gn")
+  " check we are now not in the sandbox
+  call setwinvar(1, 'myvar', 1)
+  bwipe!
+endfunc
+