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)

src/testdir/test_undo.vim

@@ -440,5 +440,10 @@ funct Test_undofile()
   " 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