When the code for handling the vertical centering of the collapse button

for diff fragments was moved into RB.CenteredElementManager, the

centering logic regressed. The new logic expected that the element would

center relative to a single container, but the old logic carefully

calculated a top and bottom container for the centering, allowing the

collapse button to appear correctly centered across several diff chunks.
RB.CenteredElementManager now allows for a top and bottom container to

be specified, and will use both of those for the centering logic. If

only a top container is specified, it will be used for both.
This fixes the display of the collapse button when expanding diffs.

Expanded several diff fragments in both directions, repeatedly, and

saw that the collapse button was always properly vertically centered in

relation to the entire visible diff area, and to the screen.