pack-objects: specify --index-version argument help explicitly

Wrap both placeholders in the argument help string in angular brackets
to signal that users needs replace them with some actual value.  Use the
flag PARSE_OPT_LITERAL_ARGHELP to prevent parseopt from adding another
pair.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -2941,9 +2941,9 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "all-progress-implied",
 			 &all_progress_implied,
 			 N_("similar to --all-progress when progress meter is shown")),
-		{ OPTION_CALLBACK, 0, "index-version", NULL, N_("version[,offset]"),
+		{ OPTION_CALLBACK, 0, "index-version", NULL, N_("<version>[,<offset>]"),
 		  N_("write the pack index file in the specified idx format version"),
-		  0, option_parse_index_version },
+		  PARSE_OPT_LITERAL_ARGHELP, option_parse_index_version },
 		OPT_MAGNITUDE(0, "max-pack-size", &pack_size_limit,
 			      N_("maximum size of each output pack file")),
 		OPT_BOOL(0, "local", &local,