Update parent/child revision timeline messages to use modern language ("parent revision")
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.
Reviewed By: amckinley
Maniphest Tasks: T13151
Differential Revision: https://secure.phabricator.com/D19514