`jj undo` and `jj op revert` used to be aliases. Now that their
semantics are planned to diverge, we need to think about which of the
existing tests belongs to which of the commands. Since `jj op revert`
will keep the current low-level semantics, all of the tests naturally
belong to it. `jj undo` will incrementally grow a new set of tests
together with its new, higher-level functionality. Some tests are
duplicated, in anticipation that they will be meaningful for `jj undo`
as well.
The moved tests are left as-is, except for any references to "undo" are
replaced by "revert" and invocations of `jj undo` are replaced by
invocations of `jj op revert`.
