Update parent/child revision timeline messages to use modern language ("parent revision")

Summary:
See PHI746. See also T11833, perhaps. Ref T13151.

Long ago, parent revisions were called "dependent revisions". This was changed to "parent revisions" in the action UI to improve clarity, but not changed in the timeline stories.

Update the timeline stories to use the same language the actions in the UI use.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19514