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