The number of CPUs is useful information for creating a thread pool or a
number of workers, but it's not really about threading directly.  Evict
it from the thread file

Our git_packbuilder__cache_lock function returns a value; use git__noop.

`git_buf_sanitize` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

We've accumulated quite some functions which are never used outside of
their respective code unit, but which are lacking the `static` keyword.
Add it to reduce their linkage scope and allow the compiler to optimize
better.

git_packbuilder_write: Allow setting path to NULL to use the default path

Propagate failures caused by pool initialization errors.

If given a NULL path, write to the object path of the repository.

Add tests for the new behavior.