Update line count after attaching commit diff

Summary: We should probably do this also in differential.creatediff but it's not a big deal because we later call differential.updaterevision which does this using DifferentialRevisionEditor.

Test Plan: None.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3263