Update: in the October 2017 release, “inline change review” was introduced, where you can easily discard a particular change in the same way as you would in IntelliJ-based IDEs:
Previously, this was only possible with the Open Changes UI (located in the upper right of editors 
). Alternatively, you can run the Git: Open Changes command.
In that view, the More button (
) offers several options for operating on individual lines, among them Revert Selected Ranges.
