I feel the original -------/+++++++ pair is slightly confusing because
each half can be a separator by itself. I don't know what character other
than '-'/'+' is preferred, but let's pick '%' (for "mod") per @martinvonz
suggestion.
Add the `jj interdiff` command for comparing only the diffs of commits.
Its args are identical to that of `jj diff`, minus `--revision` (because
interdiff always requires two commits).
Like `jj obslog -p`, Changes introduced by intervening commits are
ignored by rebasing `--from` onto `--to` 's parents.