git clone should not try to clone submodules and surface errors properly

[Panaetius]

The code in https://github.com/SwissDataScienceCenter/renku-python/blob/master/renku/core/util/git.py#L825-L833 will try to clone a repo with a branch, and if it fails, try again without the branch in case the ref was not a branch but a commit/tag instead, with a checkout after the clone.

As things currently are, if a user has submodules in their repo which are either private or use a git ssh url (because the core-svc does not have the ssh executable installed), cloning will fail, but the second call to clone will swallow the error from the first try, but will fail because the target directory is now non-empty from the failed first call.

We should:

• clone without cloning submodules
• clean the folder if the first clone failed and it's now non-empty
• make sure the correct error is returned to the user