get_patch_filename(): drop "just-numbers" hack

The function chooses from three operating modes (format using the
subject, the commit, or just number) based on NULL-ness of two of
its parameters, which is an ugly hack for sharing only a bit of
code.

Separate out the "just numbers" part out to the callers.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -684,7 +684,10 @@ static int reopen_stdout(struct commit *commit, const char *subject,
 			strbuf_addch(&filename, '/');
 	}
 
-	get_patch_filename(&filename, commit, subject, rev);
+	if (rev->numbered_files)
+		strbuf_addf(&filename, "%d", rev->nr);
+	else
+		get_patch_filename(&filename, commit, subject, rev);
 
 	if (!quiet)
 		fprintf(realstdout, "%s\n", filename.buf + outdir_offset);