old_contents: class Errors:
      '''enum-like values to use as exit codes'''
      USAGE_ERROR = 1
      DEPENDENCY_NOT_FOUND = 2
      VERSION_ERROR = 3
      GIT_CONFIG_ERROR = 4
      STRANGE_ENVIRONMENT = 5
      EATING_WITH_STAGED_CHANGES = 6
      BAD_GIT_DIRECTORY = 7
      BRANCH_EXISTS_ON_INIT = 8
      NO_SUCH_BRANCH = 9
      REPOSITORY_NOT_INITIALIZED = 10
      GIT_DIRECTORY_RELATIVE = 11
      FINDING_HEAD = 12
      BAD_TREE = 13
      GIT_DIRECTORY_MISSING = 14
  
new_contents: class Errors:
      '''enum-like values to use as exit codes'''
      USAGE_ERROR = 1
      DEPENDENCY_NOT_FOUND = 2
      VERSION_ERROR = 3
      GIT_CONFIG_ERROR = 4
      STRANGE_ENVIRONMENT = 5
      EATING_WITH_STAGED_CHANGES = 6
      BAD_GIT_DIRECTORY = 7
      BRANCH_EXISTS_ON_INIT = 8
      NO_SUCH_BRANCH = 9
      REPOSITORY_NOT_INITIALIZED = 10
      GIT_DIRECTORY_RELATIVE = 11
      FINDING_HEAD = 12
      BAD_TREE = 13
      GIT_DIRECTORY_MISSING = 14
      DIRECTORY_TO_BACKUP_MISSING = 15
  
subject: Add a new error code