vim-patch:9.1.0761: :cd completion fails on Windows with backslash in path (#30703)

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: vim/vim#15643
closes: vim/vim#15808

1a31c430bb

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/fileio.c

@@ -3618,6 +3618,7 @@ bool match_file_list(char *list, char *sfname, char *ffname)
 /// @param pat_end     first char after pattern or NULL
 /// @param allow_dirs  Result passed back out in here
 /// @param no_bslash   Don't use a backward slash as pathsep
+///                    (only makes a difference when BACKSLASH_IN_FILENAME in defined)
 ///
 /// @return            NULL on failure.
 char *file_pat_to_reg_pat(const char *pat, const char *pat_end, char *allow_dirs, int no_bslash)