vim-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 vim/vim#4392,
            closes vim/vim#4394)
e9ebc9a91c

src/nvim/testdir/test_undo.vim

@@ -389,6 +389,11 @@ 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