R1:0f919ecd3ccf

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