chunk-format: create read chunk API
Add the capability to read the table of contents, then pair the chunks
with necessary logic using read_chunk_fn pointers. Callers will be added
in future changes, but the typical outline will be:
1. initialize a 'struct chunkfile' with init_chunkfile(NULL).
2. call read_table_of_contents().
3. for each chunk to parse,
a. call pair_chunk() to assign a pointer with the chunk position, or
b. call read_chunk() to run a callback on the chunk start and size.
4. call free_chunkfile() to clear the 'struct chunkfile' data.
We are re-using the anonymous 'struct chunkfile' data, as it is internal
to the chunk-format API. This gives it essentially two modes: write and
read. If the same struct instance was used for both reads and writes,
then there would be failures.
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>