merge-base: use OPT_CMDMODE and clarify the command line parsing

The --octopus, --independent and --is-ancestor are mutually
exclusive command modes (in addition to not giving any of these
options), so represent them as such using the recent OPT_CMDMODE
facility available since 11588263 (parse-options: add OPT_CMDMODE(),
2013-07-30), which is in v1.8.4-82-g366b80b.  --all is compatible
only with plain vanilla mode and --octopus mode, and the minimum
number of arguments the command takes depends on the command modes,
so these are now separately checked in each command mode.

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

builtin/merge-base.c

@@ -90,32 +90,38 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	struct commit **rev;
 	int rev_nr = 0;
 	int show_all = 0;
-	int octopus = 0;
-	int reduce = 0;
-	int is_ancestor = 0;
+	int cmdmode = 0;
 
 	struct option options[] = {
 		OPT_BOOL('a', "all", &show_all, N_("output all common ancestors")),
-		OPT_BOOL(0, "octopus", &octopus, N_("find ancestors for a single n-way merge")),
-		OPT_BOOL(0, "independent", &reduce, N_("list revs not reachable from others")),
-		OPT_BOOL(0, "is-ancestor", &is_ancestor,
-			 N_("is the first one ancestor of the other?")),
+		OPT_CMDMODE(0, "octopus", &cmdmode,
+			    N_("find ancestors for a single n-way merge"), 'o'),
+		OPT_CMDMODE(0, "independent", &cmdmode,
+			    N_("list revs not reachable from others"), 'r'),
+		OPT_CMDMODE(0, "is-ancestor", &cmdmode,
+			    N_("is the first one ancestor of the other?"), 'a'),
 		OPT_END()
 	};
 
 	git_config(git_default_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, merge_base_usage, 0);
-	if (!octopus && !reduce && argc < 2)
-		usage_with_options(merge_base_usage, options);
-	if (is_ancestor && (show_all || octopus || reduce))
-		die("--is-ancestor cannot be used with other options");
-	if (is_ancestor)
+
+	if (cmdmode == 'a') {
+		if (argc < 2)
+			usage_with_options(merge_base_usage, options);
+		if (show_all)
+			die("--is-ancestor cannot be used with --all");
 		return handle_is_ancestor(argc, argv);
-	if (reduce && (show_all || octopus))
-		die("--independent cannot be used with other options");
+	}
 
-	if (octopus || reduce)
-		return handle_octopus(argc, argv, reduce, show_all);
+	if (cmdmode == 'r' && show_all)
+		die("--independent cannot be used with --all");
+
+	if (cmdmode == 'r' || cmdmode == 'o')
+		return handle_octopus(argc, argv, cmdmode == 'r', show_all);
+
+	if (argc < 2)
+		usage_with_options(merge_base_usage, options);
 
 	rev = xmalloc(argc * sizeof(*rev));
 	while (argc-- > 0)