patch 8.1.1352: undofile() reports wrong name

Problem:    Undofile() reports wrong name. (Francisco Giordano)
Solution:   Clean up the name before changing path separators. (closes #4392,
            closes #4394)
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index 7c93e23..ba1c861 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -440,5 +440,10 @@
   " Test undofile() with 'undodir' set to a non-existing directory.
   call assert_equal('', undofile('Xundofoo'))
 
+  if isdirectory('/tmp')
+    set undodir=/tmp
+    call assert_equal('/tmp/%tmp%file', undofile('///tmp/file'))
+  endif
+
   set undodir&
 endfunc