Parsimonious buffer updates

[tejasvi]

Update only the lines which need updating. Prevents unnecessary computations for plugins which listen to buffer change events given the common case is typing already formatted code.

[rhysd]

I think this would cause performance issue when the source is very large (>10000 lines). Checking string and calling setline() which triggers buffer change event at each line. I don't know what 'unnecessary computations for plugins' means exactly, but how about adding :noautocmd to :execute?

execute 'noautocmd' len(splitted) .',$delete' '_'