pack-write: pass hash_algo to `write_idx_file()`

pack-write: pass hash_algo to `write_idx_file()`

The `write_idx_file()` function uses the global `the_hash_algo` variable
to access the repository's hash_algo. To avoid global variable usage,
pass a hash_algo from the layers above.

Since `stage_tmp_packfiles()` also resides in 'pack-write.c' and calls
`write_idx_file()`, update it to accept a `struct git_hash_algo` as a
parameter and pass it through to the callee.

Altough the layers above could have access to the hash_algo internally,
simply pass in `the_hash_algo`. This avoids any compatibility issues and
bubbles up global variable usage to upper layers which can be eventually
resolved.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>