task_callback: A callback run after each top-level expression is evaluated.
In lbartnik/experiment: Interactive History in R

From addTaskCallback(): if the data argument was specified in the call to addTaskCallback, that value is given as the fifth argument.

update_current_commit is a part of task_callback made separate due to testing purposes.

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task_callback(expr, result, successful, printed)

update_current_commit(state, env, plot, expr)

`expr`	Expression for the top-level task.
`result`	Result of the top-level task.
`successful`	A logical value indicating whether it was successfully completed or not (always `TRUE` at present).
`printed`	A logical value indicating whether the result was printed or not.
`state`	Global state, an `environment()`; passed as a parameter for testing purposes.
`env`	Environment this commits represents.
`plot`	The last plot (see `recordPlot()`).