Ensure that we maintain the `core.repositoryFormatVersion` value instead
of always overwriting it with the default.

Provide an internal function to set the repository's `objectformat`,
both in the internal object and in the configuration.

Teach the repository about the `objectformat` extension, supporting
`sha1` always and `sha256` when the experimental sha256 support is
active.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

The experimental function signature is only available when
`GIT_EXPERIMENTAL_SHA256` is enabled.

RFC: SHA256 proof of concept

In the ownership checks implemented for CVE-2022-24765, we disallowed
users to access their own repositories when running with `sudo`.

Examine the `SUDO_UID` environment variable and allow users running
with `sudo`. This matches git's behavior.

To match git's behavior with CVE 2022-29187, validate not only the
working directory, but also the gitdir and gitlink (if it exists). This
a follow up to CVE-2022-24765 that was fixed earlier.

Allow users in the administrator group to use git configs that are owned
by administrators.