repack: move 'delta_base_offset' to 'struct pack_objects_args'

repack: move 'delta_base_offset' to 'struct pack_objects_args'

The static variable 'delta_base_offset' determines whether or not we
pass the "--delta-base-offset" command-line argument when spawning
pack-objects as a child process. Its introduction dates back to when
repack was rewritten in C, all the way back in a1bbc6c017 (repack:
rewrite the shell script in C, 2013-09-15).

'struct pack_objects_args' was introduced much later on in 4571324b99
(builtin/repack.c: allow configuring cruft pack generation, 2022-05-20),
but did not move the 'delta_base_offset' variable.

Since the 'delta_base_offset' is a property of an individual
pack-objects command, re-introduce that variable as a member of 'struct
pack_objects_args', which will enable further code movement in the
subsequent commits.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>