pack: use 64 bits for the number of objects

It is a bit too hidden why we want 64 bits when we're assigning to a 32-bit
integer later on to store the number of objects, so go back to uint32_t and cast
to uint64_t during the size calculation.

Keeping it as a 32-bit value means the min size calculation overflows or gets
truncated which can lead to issues with large packfiles.

cmake: don't include `include/git2`

Including the `include/git2` build path is a seemingly unnecessary
oversight to include the generated `experimental.h` file.

These declaration poses problems on some embedded or retro Linux systems
that deliberately disable support for wchar_t from their libc.

When the contents of an entire new directory is ignored, and `FORCE` is
specified to `git_index_add_all`, ensure that we expand the entire file
list. By default, diff will coalesce a fully ignored folder into a
single diff entry; expand it.

Ensure that when all files beneath a directory are ignored that we
add the files when FORCE is specified.

Ensure that when all files beneath a directory are ignored that we
actually ignore the files.

odb: restore `git_odb_open`

fs_path: let root run the ownership tests

Remove unused wditer variable

`git_odb_open` was erroneously removed during a refactoring; add it
back.

Found this randomly while debugging #6517

Revert #6503

The library name is correctly libgit2 (not git2) or libgit2-experimental
depending on configuration.

`git_odb_backend_loose` was erroneously removed during a refactoring;
add it back.

The `git_fs_path_owner_is_current_user` expects the root dir on unix
(`/`) to be owned by a non-current user. This makes sense unless root
(or euid == 0) is running the tests, which often happens during distro
build / packaging scripts. Allow them to run the tests.

This reverts commit 43e84e24.

This reverts commit f68b40c0.

remote: always populate old id in update tips

In b1e83cca we erroneously stopped
setting the old ID to zero; correct that.

v1.6

Pass hostkey & port to host verify callback

Co-authored-by: Stefan Karpinski <stefan@karpinski.org>

Correct the definition of "empty" in the docs for `git_repository_is_empty`