obslog: add the operation to the Obslog output
The operation is helpful when debugging a case where we don't know how we
ended up in this case while effect-flag are useful to understand what semantic
changes happens to a changeset.
Add the operation when displaying an obs-marker in the following form:
`rewritten as X using amend`
The other proposal is interesting but seems harder to internationalize and
might requires update each time we introduce new operation:
`Amended as X`
Let's use the first proposal right now.