Merge topic 'ep-update-git-hash-missing' into release-3.20

5e941a545b ExternalProject: Ensure git fetch if updating to hash we don't have yet

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Ben Boeckel <ben.boeckel@kitware.com>
Merge-request: !6109