Validate repository directory ownership (v1.4) - #6267

Provide individual file ownership checks for both the current user and
the system user, as well as a combined current user and system user
check.

Ensure that the repository directory is owned by the current user; this
prevents us from opening configuration files that may have been created
by an attacker.

Provide a mock for file ownership for testability.

Test that we prevent opening directories that are not owned by
ourselves.

Pull the global configuration loader out of the symlink check so that it
can be re-used.

Obey the `safe.directory` configuration variable if it is set in the
global or system configuration. (Do not try to load this from the
repository configuration - to avoid malicious repositories that then
mark themselves as safe.)

Introduce the `GIT_OPT_SET_OWNER_VALIDATION` option, so that users can
disable repository ownership validation.