Fix autoupdater creates additional new merge requests
Problem description
When autoupdater creates mergerequest and someone else make some changes in it, then in next run of autoupdater it creates new issue/branch/mergerequest. It should say "sorry someone else touch my job so I won't work on this project anymore".
Possible solution`
In module gitlab/update class Project has method __check_if_merge_request_can_be_edited which is used in method __get_target_merge_request. Instead this method should be used strictly in update method as condition of making update if mergerequest already exists.