midx: teach `bsearch_midx()` about incremental MIDXs

Now that the special cases callers of `bsearch_midx()` have been dealt
with, teach `bsearch_midx()` to handle incremental MIDX chains.

The incremental MIDX-aware version of `bsearch_midx()` works by
repeatedly searching for a given OID in each layer along the
`->base_midx` pointer, stopping either when an exact match is found, or
the end of the chain is reached.

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

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