Get line count before truncating Paste snippets

Summary: Fixes T12338. Resolves an issue where long pastes would be truncated before getting a line count, resulting in an inaccurate line count being returned.

Test Plan: Made a large paste, verified that it displayed the correct number of lines.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T12338

Differential Revision: https://secure.phabricator.com/D17438