This change fixes a packfile heap corruption that can happen when
interacting with multiple packfiles concurrently across multiple
threads. This is exacerbated by setting a lower mwindow open file limit.
This change:
* Renames most of the internal methods in pack.c to clearly indicate
that they expect to be called with a certain lock held, making
reasoning about the state of locks a bit easier.
* Splits the `git_pack_file` lock in two: the one in `git_pack_file`
only protects the `index_map`. The protection to `git_mwindow_file` is
now in that struct.
* Explicitly checks for freshness of the `git_pack_file` in
`git_packfile_unpack_header`: this allows the mwindow implementation
to close files whenever there is enough cache pressure, and
`git_packfile_unpack_header` will reopen the packfile if needed.
* After a call to `p_munmap()`, the `data` and `len` fields are poisoned
with `NULL` to make use-after-frees more evident and crash rather than
being open to the possibility of heap corruption.
* Adds a test case to prevent this from regressing in the future.
Fixes: #5591
