backfill: add builtin boilerplate

In anticipation of implementing 'git backfill', populate the necessary files
with the boilerplate of a new builtin. Mark the builtin as experimental at
this time, allowing breaking changes in the near future, if necessary.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/backfill.c

@@ -0,0 +1,28 @@
+#include "builtin.h"
+#include "config.h"
+#include "parse-options.h"
+#include "repository.h"
+#include "object.h"
+
+static const char * const builtin_backfill_usage[] = {
+	N_("git backfill [<options>]"),
+	NULL
+};
+
+int cmd_backfill(int argc, const char **argv, const char *prefix, struct repository *repo)
+{
+	struct option options[] = {
+		OPT_END(),
+	};
+
+	show_usage_if_asked(argc, argv, builtin_backfill_usage[0]);
+
+	argc = parse_options(argc, argv, prefix, options, builtin_backfill_usage,
+			     0);
+
+	repo_config(repo, git_default_config, NULL);
+
+	die(_("not implemented"));
+
+	return 0;
+}