patch 9.1.0761: :cd completion fails on Windows with backslash in path

Problem:  :cd completion fails on Windows with backslash in path
Solution: switch no_bslash argument to FALSE in file_pat_to_reg_pat()

Note: only fixes the problem on Windows. For Unix, we still need to
escape backslashes since those are taken as regex atoms (and could be
invalid regex atoms).

fixes: #15643
closes: #15808

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/fileio.c

@@ -5570,6 +5570,8 @@ match_file_list(char_u *list, char_u *sfname, char_u *ffname)
  * allow_dirs, otherwise FALSE is put there -- webb.
  * Handle backslashes before special characters, like "\*" and "\ ".
  *
+ * no_bslash only makes a difference, when BACKSLASH_IN_FILENAME is defined
+ *
  * Returns NULL when out of memory.
  */
     char_u *