Fix devel/qgit with newer git versions

repo-config is no longer supported by any upstream-supported
git version. Instead, use git config (which is implicitly
git config --file, corresponding to the earlier config-in-repo).

PR: 196745
Submitted by: Adriaan de Groot <groot@kde.org>, swills
Reported by: swills, groot@kde.org
Reviewed by: tcberner, rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D9861