refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/autocmd.c

@@ -2214,7 +2214,7 @@ bool has_autocmd(event_T event, char *sfname, buf_T *buf)
 #ifdef BACKSLASH_IN_FILENAME
   // Replace all backslashes with forward slashes. This makes the
   // autocommand patterns portable between Unix and Windows.
-  sfname = vim_strsave(sfname);
+  sfname = xstrdup(sfname);
   forward_slash(sfname);
   forward_slash(fname);
 #endif