Be more explicit in rendering context links in Differential

Summary: See T368. The current rendering result can cause some confusion for the
first/last chunks, make their behavior more explicit.
Test Plan:

• Clicked various "show more" links on a bunch of top/bottom/middle omitted

context blocks in a variety of diffs.

• Located a @generated shielded file and verified the initial render is

correct when the entire file is default-hidden.

Reviewed By: avitaloliver
Reviewers: avitaloliver, jungejason, tuomaspelkonen, aran
CC: aran, avitaloliver
Differential Revision: 744