CI: clean repo before updating master branch

This recently failed:

+ git checkout FETCH_HEAD
11:46:08.046  error: Your local changes to the following files would be overwritten by checkout:
11:46:08.046  	deploy/crd/pmem-csi.intel.com_pmemcsideployments.yaml
11:46:08.046  Please commit your changes or stash them before you switch branches.

It's not clear what those changes where (file seems to be up-to-date). "git
diff" will clarify this. But probably the local changes can be ignored.

Jenkinsfile

@@ -306,6 +306,8 @@ pipeline {
                     // All tests have passed on the "devel" branch, we can now fast-forward "master" to it.
                     sh '''
 head=$(git rev-parse HEAD) &&
+git diff &&
+git reset --hard &&
 git fetch origin master &&
 git checkout FETCH_HEAD &&
 git merge --ff-only $head &&