Fix TextContent::preSaveTransform() return type

TextContent::preSaveTransform should return an object of the same class
as $this, not a WikitextContent.

Apparently this was never noticed because most code paths rtrim the
input and TextContent::preSaveTransform returns $this if the rtrim is a
no-op on the input.

Bug: 58451
Change-Id: I9f8f78d52448d1ba055a654a12ff4fad86756110
Merge "Fix TextContent::preSaveTransform() return type"