midx: use context in write_midx_pack_names()

midx: use context in write_midx_pack_names()

In an effort to align the write_midx_internal() to use the chunk-format
API, start converting chunk writing methods to match chunk_write_fn. The
first case is to convert write_midx_pack_names() to take "void *data".
We already have the necessary data in "struct write_midx_context", so
this conversion is rather mechanical.

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