Package as a native image to speed up merges

[wetneb]

When merging branches, the merge driver is invoked once per file. This means many independent process will be created. Because starting a JVM takes a bit of time, it might be possible to speed this up using GraalVM's native image support.

This would create an executable file that could be used as merge driver, with potentially faster startup (and likely also faster execution).

[slarse]

That's a cool idea, I've added some thoughts in #481