midx: return success/failure in chunk write methods

midx: return success/failure in chunk write methods

Historically, the chunk-writing methods in midx.c have returned the
amount of data written so the writer method could compare this with the
table of contents. This presents with some interesting issues:

1. If a chunk writing method has a bug that miscalculates the written
   bytes, then we can satisfy the table of contents without actually
   writing the right amount of data to the hashfile. The commit-graph
   writing code checks the hashfile struct directly for a more robust
   verification.

2. There is no way for a chunk writing method to gracefully fail.
   Returning an int presents an opportunity to fail without a die().

3. The current pattern doesn't match chunk_write_fn type exactly, so we
   cannot share code with commit-graph.c

For these reasons, convert the midx chunk writer methods to return an
'int'. Since none of them fail at the moment, they all return 0.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>