vim-patch:8.1.0219: expanding ## fails to escape backtick

Problem:    Expanding ## fails to escape backtick.
Solution:   Escape a backtick in a file name. (closes vim/vim#3257)
2c8c681bfc

src/nvim/ex_docmd.c

@@ -8735,7 +8735,7 @@ static char_u *arg_all(void)
 #ifndef BACKSLASH_IN_FILENAME
             || *p == '\\'
 #endif
-            ) {
+            || *p == '`') {
           // insert a backslash
           if (retval != NULL) {
             retval[len] = '\\';