Merge branch '12622-backup-restore-doesn-t-clear-cache-resulting-in-missing-branches-and-tags' into 'master'

Fix: Backup restore doesn't clear cache

## What does this MR do?

Clears cache when we do backup restoring. Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12622



## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/12622

See merge request !7019