extdiff: add 3-way diff for merge changesets This adds 3-way diff for merge changesets (using -c) and for diffing the working directory context against two parents. To enable it, use the new magic value '$parent2' in the argument line. In order to work, your differ must support that the second parent argument is left out; this will happen in 2-way mode. Default arguments are as before, without enabling 3-way mode, ensuring backwards compatibility. This also fixes a problem when diffing a merge changeset with a single file change. Extdiff would sometimes do the wrong thing in that situation.