Fixes #197 GitLab CI does not clean git repositories within git repositories

Merged username-removed-197748 requested to merge patrik-lermon/gitlab-ci-multi-runner:master into master

If GitLab CI is configured to utilize git fetch instead of git clone it will not clean out any subdirectory containing a .git directory.

From git help clean :

-f [...] Git will refuse to delete directories with .git sub directory or file unless a second -f is given. [...]

How git clean is utilized right now (in shells/{cmd,bash,powershell}.go) is:

git clean -fdx

I suggest to change it to:

git clean -ffdx

to make subdirectories containing git repositories being cleaned out by default.