Investigate if "concat is needed"

[zimmski]

https://github.com/akovaski/go-diff/commit/a0c8f96cfc5c7ee619b3d18e7fbca09d21ce3000 had the idea of removing "concat" which reduces the code but it might be faster to have the functionality.

So, benchmark it. If the current concat is faster than just appending runes, document it. If not, remove it.

[zimmski]

https://gist.github.com/zimmski/bdf2419c78b54abf944c145deb23d2b9 concatenating once gave a very small edge to the copy version, reusing the slice gave a huge edge to the append version.