Avoid crashing when run in a bare git repo that somehow contains an index file

Avoid crashing when run in a bare git repo that somehow contains an index file

Do not populate the keys database with associated files,
because a bare repo has no working tree, and so it does not make sense to
populate it.

Queries of associated files in the keys database always return empty lists
in a bare repo, even if it's somehow populated. One way it could be
populated is if a user converts a non-bare repo to a bare repo.

Note that Git.Config.isBare does a string comparison, so this is not free!
But, that string comparison is very small compared to a sqlite query.

Sponsored-by: Erik Bjäreholt on Patreon