merge.mergestate: add support for persisting a custom merge driver A 'merge driver' is a coordinator for the overall merge process. It will be able to control: - tools for individual files, much like the merge-patterns configuration does today - tools that can work across groups of files - the ordering of file resolution - resolution of automatically generated files - adding and removing additional files to and from the dirstate Since it is a critical part of the merge process, it really is part of the merge state. This is a lowercase character (i.e. optional) because ignoring this is fine for older versions of Mercurial -- however, if there are any files that are specially treated by the driver, we should abort. That will happen in upcoming patches. There is a potential security issue with storing the merge driver in the merge state. See the inline comments for more details.