[Feature Request] Allow global git config on gitlab.rb file

Users should be able to set global git configuration using the gitlab.rb file instead of /var/opt/gitlab/.gitconfig, because they would get overwritten, if they set configurations there.

It would be nice if a custom .gitconfig file could be added as a setting in gitlab.rb. That way there is no need to allow every git setting to be configurable in there.

/cc @marin @JobV