sequencer: cleanup for gcc warning in non developer mode

as shown by: sequencer.c: In function ‘write_basic_state’: sequencer.c:2392:37: warning: zero-length gnu_printf format string [-Wformat-zero-length] write_file(rebase_path_verbose(), ""); where write_file will create an empty file if told to write an empty string as can be inferred by the previous call the somehow more convoluted syntax works around the issue by providing a non empty format string and is already being used for the abort safety file since 1e41229d96 ("sequencer: make sequencer abort safer", 2016-12-07) Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>