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>
diff --git a/src/findfile.c b/src/findfile.c
index 4310a50..b6ee092 100644
--- a/src/findfile.c
+++ b/src/findfile.c
@@ -2362,7 +2362,7 @@
     file_pattern[0] = '*';
     file_pattern[1] = NUL;
     STRCAT(file_pattern, pattern);
-    pat = file_pat_to_reg_pat(file_pattern, NULL, NULL, TRUE);
+    pat = file_pat_to_reg_pat(file_pattern, NULL, NULL, FALSE);
     vim_free(file_pattern);
     if (pat == NULL)
 	return;