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
Maniphest Tasks: T12338
Differential Revision: https://secure.phabricator.com/D17438