Re: [PATCH v2 03/10] reftable/basics: provide new `reftable_buf` interface

> Implement a new `reftable_buf` interface that will replace Git's own
> `strbuf` interface. This is done due to three reasons:
>
>   - The `strbuf` interfaces do not handle memory allocation failures and
>     instead causes us to die. This is okay in the context of Git, but is
>     not in the context of the reftable library, which is supposed to be
>     usable by third-party applications.
>
>   - The `strbuf` interface is quite deeply tied into Git, which makes it
>     hard to use the reftable library as a standalone library. Any
>     dependent would have to carefully extract the relevant parts of it
>     to make things work, which is not all that sensible.
>
>   - The `strbuf` interface does not use the pluggable allocators that
>     can be set up via `refatble_set_alloc()`.

> +/*
> + * Add the given bytes to the buffer. Returns 0 on success,
> + * REFTABLE_OUT_OF_MEMORY_ERROR on allocation failure.
> + */
> +int reftable_buf_add(struct reftable_buf *buf, const void *data, size_t len);