vim-patch:8.1.1769: 'shellslash' is also used for completion

Problem:    'shellslash' is also used for completion.
Solution:   Add the 'completeslash' option. (Yasuhiro Matsumoto, closes vim/vim#3612)
ac3150d385

src/nvim/ex_getln.c

@@ -5052,6 +5052,21 @@ ExpandFromContext (
     ret = expand_wildcards_eval(&pat, num_file, file, flags);
     if (free_pat)
       xfree(pat);
+#ifdef BACKSLASH_IN_FILENAME
+    if (p_csl[0] != NUL) {
+      for (int i = 0; i < *num_file; i++) {
+        char_u *ptr = (*file)[i];
+        while (*ptr != NUL) {
+          if (p_csl[0] == 's' && *ptr == '\\') {
+            *ptr = '/';
+          } else if (p_csl[0] == 'b' && *ptr == '/') {
+            *ptr = '\\';
+          }
+          ptr += utfc_ptr2len(ptr);
+        }
+      }
+    }
+#endif
     return ret;
   }