Give PhutilRope and LinesOfALarge a couple new capabilities

Summary:
Ref T10186. To fix that, I need to stream files out of commands like git cat directly into file storage.

This adds two capabilities to support this:

• LinesOfALarge can now just ignore delimiters and stream all the data directly.
• PhutilRope can now produce a prefix of a specific length (this is used to produce a chunk-sized prefix).

Test Plan: See next revision.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10186

Differential Revision: https://secure.phabricator.com/D15071