GIT_STRATEGY fetch does not correctly update submodule tags

1. git submodule update doesn't fetch anything if your submodules are up-to-date
2. So when using the GIT_STRATEGY fetch and the runner already checked out the submodule at the same commit, nothing ist done
3. If, however, somebody pushed a tag in the meantime to the submodule, the runner will not pick up that tag

The solution could be:

git submodule foreach --recursive 'git fetch --tags'

maybe even with -f for force-pushed tags? Otherwise, they will fail!

Relevant links:

• The foreach trick: https://stackoverflow.com/a/12284659/1531708
• A hint to the "no fetch if up-to-date" problem from the same SO question as above: https://stackoverflow.com/a/7504682/1531708
• Insights into different behaviors of --tags in conjunction with fetch or pull: https://stackoverflow.com/a/20608181/1531708

Proposal

To do this we need to update writeSubmoduleUpdateCmd to call for the tags.