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>

bisect.h

@@ -9,7 +9,14 @@ extern struct commit_list *filter_skipped(struct commit_list *list,
 					  struct commit_list **tried,
 					  int show_all);
 
+/*
+ * The "show_all" parameter should be 0 if this function is called
+ * from outside "builtin-rev-list.c" as otherwise it would use
+ * static "revs" from this file.
+ */
 extern int show_bisect_vars(struct rev_info *revs, int reaches, int all,
 			    int show_all, int show_tried);
 
+extern int bisect_next_vars(const char *prefix);
+
 #endif