Re: [PATCH 2/4] builtin/repack.c: pass "cruft_expiration" to `write_cruft_pack`

> `builtin/repack.c`'s `write_cruft_pack()` is used to generate the cruft
> pack when `--cruft` is supplied. It uses a static variable
> "cruft_expiration" which is filled in by option parsing.
>
> A future patch will add an `--expire-to` option which allows `git
> repack` to write a cruft pack containing the pruned objects out to a
> separate repository. In order to implement this functionality, some
> callers will have to pass a value for `cruft_expiration` different than
> the one filled out by option parsing.
>
> Prepare for this by teaching `write_cruft_pack` to take a
> "cruft_expiration" parameter, instead of reading a single static
> variable.
>
> The (sole) existing caller of `write_cruft_pack()` will pass the value
> for "cruft_expiration" filled in by option parsing, retaining existing
> behavior. This means that we can make the variable local to
> `cmd_repack()`, and eliminate the static declaration.