topic: fix compatibility issues caused because of change in transaction API In 36f08ae87ef687be53a59bd87376bcfbe4479205 in core mercurial, `_validator` attribute to transaction class was removed and a dict was introduced. It added a `addvalidator()` function to transaction class which can be used to register multiple validator callbacks. This updates the code to use `addvalidator()` when `_validator` attribute is not present.

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