Use shallow git clones in tests

[fsimonis]

The systemtests know which branch to checkout and don't need to access the git history.

Thus we can reduce the clone-time by using shallow git clones:

git clone --depth 1 --branch <branch> <repository> <directory>

This will break git describe for detecting the preCICE revision. So either we accept it, or we accept the longer clone times for the preCICE library itself.

[BenjaminRodenberg]

This will break git describe for detecting the preCICE revision. So either we accept it, or we accept the longer clone times for the preCICE library itself.

Does this mean that the preCICE revision in the log will be missing? I think it is very useful to know the preCICE revision in the systemtests when trying to find bugs. Until we have a rigorous strategy for labelling the docker images themselves with the revision they contain (e.g. #138 or #193) I would vote against sacrificing the revision info in favour of quicker building.

[fsimonis]

The core library is a special case worth mentioning and I agree that the revision information should stay intakt.

All other git clones for adapters, bindings and tutorial data should be shallow though.