midx: build `keep_hashes` array in order

midx: build `keep_hashes` array in order

Instead of filling the keep_hashes array using reverse indexing (e.g.,
`keep_hashes[count - i - 1]`) while traversing linked lists forward,
collect linked list nodes into a temporary `layers` array and then
iterate it backwards to fill `keep_hashes` sequentially.

This makes the filling logic easier to follow, since each segment of the
array is filled with a simple forward-marching index. Moreover, this
change prepares us for a subsequent commit that will switch to using a
`strvec`.

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