safe.directory: document and check that it's ignored in the environment

safe.directory: document and check that it's ignored in the environment

The description of 'safe.directory' mentions that it's respected in
the system and global configs, and ignored in the repository config
and on the command line, but it doesn't mention whether it's respected
or ignored when specified via environment variables (nor does the
commit message adding 'safe.directory' [1]).

Clarify that 'safe.directory' is ignored when specified in the
environment, and add tests to make sure that it remains so.

[1] 8959555cee (setup_git_directory(): add an owner check for the
                top-level directory, 2022-03-02)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>