Merge branch 'gitaly-mirroring-prep' into 'master'
Gitaly mirroring migration prep
Closes gitaly#816
See merge request gitlab-org/gitlab-ce!15775
(cherry picked from commit 6930fa31)
885a4da2 Add feature flag to use gitaly-ssh mirroring when cloning internal repos
95009cef Unify mirror addition operations to prepare for Gitaly migration