DairikiDiff: Optimise method WordLevelDiff._split()

_split() copied two arrays N times, where N is number of lines in diff.
This was done by $a = array_merge($a, ...);
Instead of doing this, new words are appended to the end of array
using []= syntax.

Bug: 47989
Change-Id: I41338a2a82fbc20d7511f4c79581880febeeeea5