reftable/block: reuse uncompressed blocks

The reftable backend stores reflog entries in a compressed format and thus needs to uncompress blocks before one can read records from it. For each reflog block we thus have to allocate an array that we can decompress the block contents into. This block is being discarded whenever the table iterator moves to the next block. Consequently, we reallocate a new array on every block, which is quite wasteful. Refactor the code to reuse the uncompressed block data when moving the block reader to a new block. This significantly reduces the number of allocations when iterating through many compressed blocks. The following measurements are done with `git reflog list` when listing 100k reflogs. Before: HEAP SUMMARY: in use at exit: 13,473 bytes in 122 blocks total heap usage: 45,755 allocs, 45,633 frees, 254,779,456 bytes allocated After: HEAP SUMMARY: in use at exit: 13,473 bytes in 122 blocks total heap usage: 23,028 allocs, 22,906 frees, 162,813,547 bytes allocated Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>