Fix code owner functionality
There was a problem with the script such that in some cases to many code owners were informed. When in between the creation of the MR and the runtime of the script the master branch of the official repository was updated the code owners of the MR changes as well as the code owners of the updated code from the official repository wer informed. To avoid sending information for code which is already implemented in the official repository the local working copy is rebased before extracting the changed files.
Fix user name of Sergey Gorbunov in CODEOWNERS file. Sergey has two GitLab accounts and the wrong was chosen.