effectflag: allow to save effect of change in obsmarkers Obsmarkers lack a precious information, what are the changes between the source and the destination. This could be several thing: - The description has changed, with a amend -e for example - The parent has changed, with a rebase for example - The content has changed, with a amend for example - The meta, like the user, has change, with a amend -u for example Or any combination of them. We hijack the createobsmarkers in order to compute the modification and store the effect flag which is basically a bitfield of the changes list above and we store it in the metadata for the moment. The advantage of computing the flag just before creating the obs marker is that all commands will immediately be compatible and we can start working on the UI display of the flag. As a future optimization, the commands will be able to pass the flag directly or data to help computing them, for example amend knows if the description has changed or rebase now if the parent has changed, virtually removing the overhead. To start simple, this patch add the detection and storage of the effect flag when the description has changed. And hide it behind an experimental flag disabled by default 'experimental.evolution.effect-flags'. More flag will follow.

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