Make git-mv a builtin

This also moves add_file_to_index() to read-cache.c. Oh, and while
touching builtin-add.c, it also removes a duplicate git_config() call.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git.c

@@ -202,6 +202,7 @@ static void handle_internal_command(int argc, const char **argv, char **envp)
 		{ "update-ref", cmd_update_ref },
 		{ "fmt-merge-msg", cmd_fmt_merge_msg },
 		{ "prune", cmd_prune },
+		{ "mv", cmd_mv },
 	};
 	int i;