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.