midx: pass custom '--base' through incremental writes

The 'multi-pack-index' builtin parses '--base' for incremental writes, but the normal write path does not pass that value through to `write_midx_file()`. As a result, something like: $ git multi-pack-index write --incremental --base=<base> behaves as if no custom base had been given (unless the caller used the '--stdin-packs' path). Thread the parsed base through `write_midx_file()`, and update the repack caller to pass NULL for the new argument where no custom base selection is needed. This exposes a pre-existing problem in incremental writes with custom bases: the writer skips packs from the full existing MIDX chain, even when the caller selected an older base or no base at all. The affected t5334 cases fail while trying to write MIDX bitmaps. The detached layer omits packs above the selected base, and thus the resulting MIDX does not have a reachability closure, making it impossible to generate reachability bitmaps. Mark those tests as expected failures accordingly. The following commit will fix the broken behavior and restore these tests. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>