The new git_repository_workdir_path function does error checking on
working directory inputs on Windows; use it to construct paths within
working directories.

Also repurposed an unused function and deleted another one.

This change makes that anything that calls `git_diff__from_iterators`
(any of the `git_diff_xxx` functions) only need to parse the
`.gitmodules` file once. This can be avoided by calling
`git_repository_submodule_cache_all(...)`, but we can do that safely for
the user with no change in semantics.

Fixes: #5725

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.

Propagate failures caused by pool initialization errors.

libgit2 does not use `type_t` suffixes as it's redundant; thus, rename
`git_iterator_type_t` to `git_iterator_t` for consistency.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.