We have a variety of ways that we link to specific places in the diff

viewer, and one of the most common and useful is clicking on the link

from a comment to jump straight to the comment in context. This was

broken a while back.
The fundamental cause here is that the selectAnchor() method is

checking whether the comment flag's element is shown, but

TextBasedCommentBlock defers its own show operation in order to let

layout settle. We therefore were always bailing early.
This change makes two fixes:

We now defer the whole operation for the #startAtAnchorName thing.

  This lets it run after comment flags have shown themselves.
We check the return value of selectAnchor() and verify that we

  actually did something before clearing out the saved anchor name.

Ran js-tests
Clicked on a bunch of links from diff comments and saw that they

  now properly scrolled the page.
Checked explicitly that a link to a comment on the last file on the

  page worked.