Michael Lohmann [off-list ref] writes:

Git considers all repositories as safe, if they are either
 - explicitly set in "safe.directory" config, or
 - the user owns the repo

If you are going to reroll this step, please add a few more cases to
the list above.  There are other code paths in setup.c that does not
call ensure_valid_ownership().  Treating an explicitly specified git
directory as safe is one of them (there may or may not be others, I
didn't check).