merge.mergestate: add support for persisting driver-resolved files A driver-resolved file is a file that's handled specially by the driver. A common use case for this state would be autogenerated files, the generation of which should happen only after all source conflicts are resolved. This is done with an uppercase letter because older versions of Mercurial will not know how to treat such files at all.

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.