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Add another pattern to quoted email body stripping

Summary:
Ref T3217. @hlau has an email client which quotes text with:

  ________________________________________
  From: ...

Add a regular expression to correctly detect this as quoted body text.

Test Plan: Add unit test; executed unit test.

Reviewers: btrahan, chad

Reviewed By: chad

CC: hlau, aran

Maniphest Tasks: T3217

Differential Revision: https://secure.phabricator.com/D5986