setup_git_directory(): add an owner check for the top-level directory

It poses a security risk to search for a git directory outside of the directories owned by the current user. For example, it is common e.g. in computer pools of educational institutes to have a "scratch" space: a mounted disk with plenty of space that is regularly swiped where any authenticated user can create a directory to do their work. Merely navigating to such a space with a Git-enabled `PS1` when there is a maliciously-crafted `/scratch/.git/` can lead to a compromised account. The same holds true in multi-user setups running Windows, as `C:\` is writable to every authenticated user by default. To plug this vulnerability, we stop Git from accepting top-level directories owned by someone other than the current user. We avoid looking at the ownership of each and every directories between the current and the top-level one (if there are any between) to avoid introducing a performance bottleneck. This new default behavior is obviously incompatible with the concept of shared repositories, where we expect the top-level directory to be owned by only one of its legitimate users. To re-enable that use case, we add support for adding exceptions from the new default behavior via the config setting `safe.directory`. The `safe.directory` config setting is only respected in the system and global configs, not from repository configs or via the command-line, and can have multiple values to allow for multiple shared repositories. We are particularly careful to provide a helpful message to any user trying to use a shared repository. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>