bisect--helper: implement "git bisect--helper"

This patch implements a new "git bisect--helper" builtin plumbing
command that will be used to migrate "git-bisect.sh" to C.

We start by implementing only the "--next-vars" option that will
read bisect refs from "refs/bisect/", and then compute the next
bisect step, and output shell variables ready to be eval'ed by
the shell.

At this step, "git bisect--helper" ignores the paths that may
have been put in "$GIT_DIR/BISECT_NAMES". This will be fixed in a
later patch.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-bisect--helper.c

@@ -0,0 +1,27 @@
+#include "builtin.h"
+#include "cache.h"
+#include "parse-options.h"
+#include "bisect.h"
+
+static const char * const git_bisect_helper_usage[] = {
+	"git bisect--helper --next-vars",
+	NULL
+};
+
+int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
+{
+	int next_vars = 0;
+	struct option options[] = {
+		OPT_BOOLEAN(0, "next-vars", &next_vars,
+			    "output next bisect step variables"),
+		OPT_END()
+	};
+
+	argc = parse_options(argc, argv, options, git_bisect_helper_usage, 0);
+
+	if (!next_vars)
+		usage_with_options(git_bisect_helper_usage, options);
+
+	/* next-vars */
+	return bisect_next_vars(prefix);
+}