extdiff: register the configuration generated commands with a help category Otherwise, 'extdiff' shows up under file management and the rest of the commands are at the bottom under 'Uncategorized'.

rebase: abort in-mem rebase if there's a dirty merge state In-memory merge uses the on-disk merge state, so we should not allow it run in-memory merge when the merge state is not clean. We should probably not use the on-disk merge state when running in-memory merge, but chaning that is not suitable for the stable branch. Differential Revision: https://phab.mercurial-scm.org/D5357

rebase: preserve working copy when redoing in-mem rebase on disk When in-memory rebase runs into conflicts, we retry it on disk. But before we do that, we abort the in-memory rebase. That is done because even though it's mostly in memory, there are still a few state files written (e.g. the merge state). We should make it not write those files so we don't need to abort, but for the stable branch, let's explicitly clear the state we need to clear instead of running the usual abort code. Differential Revision: https://phab.mercurial-scm.org/D5356