Factor out DifferentialDiffTableOfContentsView::renderChangesetLink()

Summary:
Links from lint errors for large diffs don't work.
This diff adds TODO for it because I am not sure how to do it.
Move of changeset links rendering to a separate method would be still useful.

Test Plan:
Display ToC of large diff, verify link.
Repeat for small diff.

Reviewers: tuomaspelkonen, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1476