bloom: prepare to discard incompatible Bloom filters

Callers use the inline `get_bloom_filter()` implementation as a thin
wrapper around `get_or_compute_bloom_filter()`. The former calls the
latter with a value of "0" for `compute_if_not_present`, making
`get_bloom_filter()` the default read-only path for fetching an existing
Bloom filter.

Callers expect the value returned from `get_bloom_filter()` is usable,
that is that it's compatible with the configured value corresponding to
`commitGraph.changedPathsVersion`.

This is OK, since the commit-graph machinery only initializes its BDAT
chunk (thereby enabling it to service Bloom filter queries) when the
Bloom filter hash_version is compatible with our settings. So any value
returned by `get_bloom_filter()` is trivially useable.

However, subsequent commits will load the BDAT chunk even when the Bloom
filters are built with incompatible hash versions. Prepare to handle
this by teaching `get_bloom_filter()` to discard filters that are
incompatible with the configured hash version.

Callers who wish to read incompatible filters (e.g., for upgrading
filters from v1 to v2) may use the lower level routine,
`get_or_compute_bloom_filter()`.

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

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