Re: [PATCH v3 5/8] reftable/record: store "val1" hashes as static arrays

> When reading ref records of type "val1", we store its object ID in an
> allocated array. This results in an additional allocation for every
> single ref record we read, which is rather inefficient especially when
> iterating over refs.
>
> Refactor the code to instead use an embedded array of `GIT_MAX_RAWSZ`
> bytes. While this means that `struct ref_record` is bigger now, we
> typically do not store all refs in an array anyway and instead only
> handle a limited number of records at the same point in time.
>
> Using `git show-ref --quiet` in a repository with ~350k refs this leads
> to a significant drop in allocations. Before:
>
>     HEAP SUMMARY:
>         in use at exit: 21,098 bytes in 192 blocks
>       total heap usage: 2,116,683 allocs, 2,116,491 frees, 76,098,060 bytes allocated
>
> After:
>
>     HEAP SUMMARY:
>         in use at exit: 21,098 bytes in 192 blocks
>       total heap usage: 1,419,031 allocs, 1,418,839 frees, 62,145,036 bytes allocated