setup: opt-out of check with safe.directory=*

With the addition of the safe.directory in 8959555ce (setup_git_directory(): add an owner check for the top-level directory, 2022-03-02) released in v2.35.2, we are receiving feedback from a variety of users about the feature. Some users have a very large list of shared repositories and find it cumbersome to add this config for every one of them. In a more difficult case, certain workflows involve running Git commands within containers. The container boundary prevents any global or system config from communicating `safe.directory` values from the host into the container. Further, the container almost always runs as a different user than the owner of the directory in the host. To simplify the reactions necessary for these users, extend the definition of the safe.directory config value to include a possible '*' value. This value implies that all directories are safe, providing a single setting to opt-out of this protection. Note that an empty assignment of safe.directory clears all previous values, and this is already the case with the "if (!value || !*value)" condition. Signed-off-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>