fix: remove trailing slashes before making directory

Remove the trailing slashes from 'undofile' and 'backupdir' before
creating directories. They cause problems on Windows which doesn't
recognize these slashes as proper path separators.

src/nvim/memline.c

@@ -1442,7 +1442,7 @@ recover_names (
  * Append the full path to name with path separators made into percent
  * signs, to dir. An unnamed buffer is handled as "" (<currentdir>/"")
  */
-char *make_percent_swname(const char *dir, char *name)
+char *make_percent_swname(const char *dir, const char *name)
   FUNC_ATTR_NONNULL_ARG(1)
 {
   char *d = NULL;