We use p->index_map.data to check whether the struct has been set up
and all the information about the index is stored there. This variable
gets set up halfway through the setup process, however, and a thread
can come along and use fields that haven't been written to yet.

Crucially, pack_entry_find_offset() needs to read the index version
(which is written after index_map) to know the offset and stride
length to pass to sha1_entry_pos(). If these values are wrong,
assertions in it will fail, as it will be reading bogus data.

Make index_version the last field to be written and switch from using
p->index_map.data to p->index_version as "git_pack_file is ready" flag
as we can use it to know if every field has been written.