midx: support custom `--base` for incremental MIDX writes

Both `compact` and `write --incremental` fix the base of the resulting
MIDX layer: `compact` always places the compacted result on top of
"from's" immediate parent in the chain, and `write --incremental` always
appends a new layer to the existing tip. In both cases the base is not
configurable.

Future callers need additional flexibility. For instance, the incremental
MIDX-based repacking code may wish to write a layer based on some
intermediate ancestor rather than the current tip, or produce a root
layer when replacing the bottommost entries in the chain.

Introduce a new `--base` option for both subcommands to specify the
checksum of the MIDX layer to use as the base. The given checksum must
refer to a valid layer in the MIDX chain that is an ancestor of the
topmost layer being written or compacted.

The special value "none" is accepted to produce a root layer with no
parent. This will be needed when the incremental repacking machinery
determines that the bottommost layers of the chain should be replaced.

If no `--base` is given, behavior is unchanged: `compact` uses "from's"
immediate parent in the chain, and `write` appends to the existing tip.

For the `write` subcommand, `--base` requires `--no-write-chain-file`. A plain
`write --incremental` appends a new layer to the live chain tip with no
mechanism to atomically replace it; overriding the base would produce a
layer that does not extend the tip, breaking chain invariants. With
`--no-write-chain-file` the chain is left unmodified and the caller is
responsible for assembling a valid chain.

For `compact`, no such restriction applies. The compaction operation
atomically replaces the compacted range in the chain file, so writing
the result on top of any valid ancestor preserves chain invariants.

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

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