Speed up single-line deltas

2e6f12725bc01011c545c99c1f1c1da6065063d9 slowed down the application of
deltas that only affect a single line. The slow-down, though trivial for
a single line, is significant for operations than separately modify
thousands of rows (such as indenting a large document).

This commit speeds up single-line deltas by avoiding unnecessary calls
to splitLine() and joinLineWithNext().