repo-settings: prepare_repo_settings only in git repos
Check whether git directory exists before adding any repo settings. If it
does not exist, BUG with the message that one cannot add settings for an
uninitialized repository. If it does exist, proceed with adding repo
settings.
Signed-off-by: Lessley Dennington <lessleydennington@gmail.com> Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>