builtin/repack.c: introduce `struct write_pack_opts`

builtin/repack.c: introduce `struct write_pack_opts`

There are various functions within the 'repack' builtin which are
responsible for writing different kinds of packs. They include:

 - `static int write_filtered_pack(...)`
 - `static int write_cruft_pack(...)`

as well as the function `finish_pack_objects_cmd()`, which is
responsible for finalizing a new pack write, and recording the checksum
of its contents in the 'names' list.

Both of these `write_` functions have a few things in common. They both
take a pointer to the 'pack_objects_args' struct, as well as a pair of
character pointers for `destination` and `pack_prefix`.

Instead of repeating those arguments for each function, let's extract an
options struct called "write_pack_opts" which has these three parameters
as member fields. While we're at it, add fields for "packdir," and
"packtmp", both of which are static variables within the builtin, and
need to be read from within these two functions.

This will shorten the list of parameters that callers have to provide to
`write_filtered_pack()`, avoid ambiguity when passing multiple variables
of the same type, and provide a unified interface for the two functions
mentioned earlier.

(Note that "pack_prefix" can be derived on the fly as a function of
"packdir" and "packtmp", making it unnecessary to store "pack_prefix"
explicitly. This commit ignores that potential cleanup in the name of
doing as few things as possible, but a later commit will make that
change.)

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