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.

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