repack: move `find_pack_prefix()` out of the builtin

repack: move `find_pack_prefix()` out of the builtin

Both callers within the repack builtin which call functions that take a
'write_pack_opts' structure have the following pattern:

    struct write_pack_opts opts = {
        .packdir = packdir,
        .packtmp = packtmp,
        .pack_prefix = find_pack_prefix(packdir, packtmp),
        /* ... */
    };
    int ret = write_some_kind_of_pack(&opts, /* ... */);

, but both "packdir" and "packtmp" are fields within the write_pack_opts
struct itself!

Instead of also computing the pack_prefix ahead of time, let's have the
callees compute it themselves by moving `find_pack_prefix()` out of the
repack builtin, and have it take a write_pack_opts pointer instead of
the "packdir" and "packtmp" fields directly.

This avoids the callers having to do some prep work that is common
between the two of them, but also avoids the potential pitfall of
accidentally writing:

    .pack_prefix = find_pack_prefix(packtmp, packdir),

(which is well-typed) when the caller meant to instead write:

    .pack_prefix = find_pack_prefix(packdir, packtmp),

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