range-diff: respect diff_option.file rather than assuming 'stdout'

The actual diffs output by range-diff respect diff_option.file, which
range-diff passes down the call-chain, thus are destination-agnostic.
However, output_pair_header() is hard-coded to emit to 'stdout'. Fix
this by making output_pair_header() respect diff_option.file, as well.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -323,7 +323,7 @@ static void output_pair_header(struct diff_options *diffopt,
 	}
 	strbuf_addf(buf, "%s\n", color_reset);
 
-	fwrite(buf->buf, buf->len, 1, stdout);
+	fwrite(buf->buf, buf->len, 1, diffopt->file);
 }
 
 static struct userdiff_driver no_func_name = {