Fix rejection of banned commits

Commit 936470ed2e9967e9e509b8472a3e1872b5e5fe77 broke the rejection of
banned commits. During the refactoring the check for banned commits
was removed. Add a new commit validator that rejects pushes that
contain banned commits.

Change-Id: I1645dd437129336fcbfb0f18ee1c801db973d482
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
4 files changed