Avoid Git.Config.updateLocation adding "/.git" to the end of the repo

Avoid Git.Config.updateLocation adding "/.git" to the end of the repo
path to a bare repo when git config is not allowed to list the configs
due to the CVE-2022-24765 fix.

That resulted in a confusing error message, and prevented the nice
message that explains how to mark the repo as safe to use.

Made isBare a tristate so that the case where core.bare is not returned can
be handled.

The handling in updateLocation is to check if the directory
contains config and objects and if so assume it's bare.
Note that if that heuristic is somehow wrong, it would construct a repo
that thinks it's bare but is not. That could cause follow-on problems,
but since git-annex then checks checkRepoConfigInaccessible, and skips
using the repo anyway, a wrong guess should not be a problem.

Sponsored-by: Luke Shumaker on Patreon