Change wording of "Show Entire File" to "Show All Context"

Summary: Fixes T10139. This clarification seems reasonable to me.

Test Plan:

• Viewed a revision with context.
• Clicked "Show All Context".
• Saw "All Context Shown".

{F1060624}

{F1060625}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10139

Differential Revision: https://secure.phabricator.com/D15009