When using "bin/bulk export --overwrite", actually overwrite the file

Summary: Depends on D19738. Ref T13210. Currently, when you use "--overwrite", we just append the new content. Instead, actually overwrite the file.

Test Plan: Used --overwrite, saw an actual clean overwrite instead of an append.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210

Differential Revision: https://secure.phabricator.com/D19739