It needlessly grabbed strings for the full text insertion range,
while it only operated on single-character insertions.

This patch bails out early if multi-character insertions are done.
Especially when opening a large file, this has a big impact, as
we'd otherwise copy the full file contents temporarily just
to figure out it's not a brace that was inserted...

This was found by massif, see e.g. the picture in this blog post:
http://milianw.de/blog/heaptrack-a-heap-memory-profiler-for-linux
The sharp peak at the highest memory consumption is the autobrace
plugin. With this patch, the peak is gone.