midx: use `strvec` for `keep_hashes`

The `keep_hashes` array in `write_midx_internal()` accumulates the
checksums of MIDX files that should be retained when pruning stale
entries from the MIDX chain. For similar reasons as in a previous
commit, rewrite this using a strvec, requiring us to pass one fewer
parameter.

Unlike the aforementioned previous commit, use a `strvec` instead of a
`string_list`, which provides a more ergonomic interface to adjust the
values at a particular index. The ordering is important here, as this
value is used to determine the contents of the resulting
`multi-pack-index-chain` file when writing with "--incremental".

Since the previous commit already builds the array in forward order, the
conversion is straightforward: replace indexed assignments with
`strvec_push()`, drop the pre-counting and `CALLOC_ARRAY()`, and
simplify cleanup via `strvec_clear()`.

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

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