format-patch: output header for empty commits

When formatting an empty commit, it is surprising that a totally empty
file is generated.  Set the flag to always print the header, matching
the behaviour of git-log.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -2097,6 +2097,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 
 	/* Always generate a patch */
 	rev.diffopt.output_format |= DIFF_FORMAT_PATCH;
+	rev.always_show_header = 1;
 
 	rev.zero_commit = zero_commit;
 	rev.patch_name_max = fmt_patch_name_max;