sequencer: cleanup for gcc warning in non developer mode

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>