Json-merger

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- In dict_merger head is always kept if root matches update.
- In dict_merger update is always kept if root matches head.

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- ``KEEP_UPDATE_CONFLICT_ON_HEAD_DELETE`` doesn't take the root into account any longer.

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- Add new typoe of conflict `INSERT`.
- `KEEP_HEAD_ENTITIES_CONFLICT_ON_NEW_UPDATE` will create INSERT conflict instead of REMOVE one.

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- Add new strategy `KEEP_HEAD_ENTITIES_CONFLICT_ON_NEW_UPDATE` which will create conflict when there is a new value in update.

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- Fix bug when DictMergeOps.keep_longest is used inside lists.

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- Add a new strategy for conflicts on head delete (contrib).