R1:fcb75d0503c6

Fix an issue where prose diffing may fail after hitting the PCRE backtracking limit

Summary:
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…