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`

This improves the documentation for `git_repository_is_empty`
which currently does not accurately describe libgit2's definition
of "empty".

It says that HEAD must point to the "unborn master branch", when
in fact, this is not the case if the repo's `init.defaultBranch`
configuration is set. If it is set, it will check that HEAD points
there. Only if it is not set does it fall back to `master`.

odb: don't unconditionally add oid_type to stream

`git_odb_stream` should have an `oid_type` to disambiguate; that's not
necessary on non-experimental SHA256 builds. Avoid unnecessary ABI
breakage for consumers and hide it behind an ifdef for now.

tests: always unset HTTP_PROXY before starting tests

The remote::httpproxy::env test (correctly) reset the environment before
running; however the other tests are also impacted by the environment
and need to have it isolated.