Add checks for proper rebase and linear history

.gitlab-ci.yml

 stages:
+  - checkRepository
   - checkFormat
 
+RebaseCheck:
+  stage: checkRepository
+  image: alpine
+  tags:
+    - docker
+  only:
+    refs:
+      - merge_requests
+    variables:
+      - $CI_MERGE_REQUEST_PROJECT_PATH == "CbmSoft/cbmroot_input" && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"
+  script:
+    # Get the upstream repository manually. I did not find any other way to have it for
+    # comparison
+    # Check if a rebase is needed
+    # If a rebase is needed stop immediately
+    - apk update && apk add git bash
+    - .scripts/connect_upstream_repo.sh $CI_MERGE_REQUEST_PROJECT_URL
+    - git fetch upstream
+    - hash1=$(git show-ref upstream/master | cut -f1 -d' ')
+    - hash2=$(git merge-base upstream/master origin/$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME)
+    - echo "${hash1}"
+    - echo "${hash2}"
+    - if [ "${hash1}" = "${hash2}" ]; then
+    -   echo "No rebase required"
+    -   exit 0
+    - else
+    -   echo "The Merge Request is not up-to-date"
+    -   echo "Rebase is required"
+    -   exit 1
+    - fi
+
+LinearHistCheck:
+  stage: checkRepository
+  image: alpine
+  tags:
+    - docker
+  only:
+    refs:
+      - merge_requests
+    variables:
+      - $CI_MERGE_REQUEST_PROJECT_PATH == "CbmSoft/cbmroot_input" && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"
+  script:
+    # Get the upstream repository manually. I did not find any other way to have it for
+    # comparison
+    # Check if the history introduced by the MR is linear (no branch loop in fork)
+    # If not linear stop immediately
+    - apk update && apk add git bash
+    - .scripts/connect_upstream_repo.sh $CI_MERGE_REQUEST_PROJECT_URL
+    - git fetch upstream
+    - count=$(git rev-list --min-parents=2 --count upstream/master..HEAD)
+    - echo "${count}"
+    - if [ 0 = "${count}" ]; then
+    -   echo "History introduced in fork is linear"
+    -   exit 0
+    - else
+    -   echo "History introduced in fork is not linear"
+    -   echo "${count} merge commits must be removed or the other new commits have to be replayed in a new branch"
+    -   exit 1
+    - fi
+
 FileFormatCheck:
   stage: checkFormat
   image: alpine