midx: use `strset` for retained MIDX files

Both `clear_midx_files_ext()` and `clear_incremental_midx_files_ext()`
build a list of filenames to keep while pruning stale MIDX files. Today
they hand-roll an array instead of using a `strset`, thus requiring us
to pass an additional length parameter, and makes lookups linear.

Replace the bare array with a `strset` which can be passed around as a
single parameter. Though it improves lookup performance, the difference
is likely immeasurable given how small the keep_hashes array typically
is.

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

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