Add git-for-each-ref: helper for language bindings

This adds a new command, git-for-each-ref.  You can have it iterate
over refs and have it output various aspects of the objects they
refer to.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git.c

@@ -231,6 +231,7 @@ static void handle_internal_command(int argc, const char **argv, char **envp)
 		{ "diff-stages", cmd_diff_stages, RUN_SETUP },
 		{ "diff-tree", cmd_diff_tree, RUN_SETUP },
 		{ "fmt-merge-msg", cmd_fmt_merge_msg, RUN_SETUP },
+		{ "for-each-ref", cmd_for_each_ref, RUN_SETUP },
 		{ "format-patch", cmd_format_patch, RUN_SETUP },
 		{ "get-tar-commit-id", cmd_get_tar_commit_id },
 		{ "grep", cmd_grep, RUN_SETUP },