histedit: report the unacceptable changeset

mdiff: don't emit a diff header for empty trivial deltas An empty trivial delta, coded as (0, 0, 0) makes the delta application do nothing, but still takes 12 bytes, while skipping it altogether works as much, without taking any space at all.

commands: get rid of empty try/finally block from _dograft This diff is purely an indentation change to clean up a block that was kept in place to make 77995317b374 easier to read.