midx-write: include packs above custom incremental base

The previous commit made '--base' take effect on the normal incremental write path, which exposed an existing assumption in our helper function `should_include_pack()`, which is that any pack already present in `ctx->m` was skipped. That is only correct for non-incremental writes. For incremental writes, `ctx->base_midx` is the boundary that should be excluded from the new layer. If the caller selects an older base, or no base at all, then packs from layers above that base have to be included in the detached layer so that its bitmap has reachability closure. Teach `should_include_pack()` to choose the MIDX used for pack exclusion based on whether or not we are performing an incremental write. When doing so, use `ctx->base_midx`, and use `ctx->m` otherwise. The t5334 cases from the previous commit can now be marked as successful. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>