Fix an issue where prose diffing may fail after hitting the PCRE backtracking limit
Fixes T13554. For certain prose diff inputs and PCRE backtracking limits, this regular expression may back track too often and fail.
A characteristic input is "x x x x ...", i.e. many sequences where (.*?)\s*\z looks like it may be able to match but actually can not.
I think writing an expression which has all the behavior we'd like without this backtracking issue isn't trivial (at least, I don't think I know how to do it offhand); just use a strategy based on "trim()" insetad, which avoids any PCRE complexities here.
Test Plan: Locally, this passes the "x x x ..." test which the previous code failed. I'm not including that test because it won't reproduce across values of "pcre.backtrac_limit", PCRE versions, etc.
Maniphest Tasks: T13554
Differential Revision: https://secure.phabricator.com/D21422