add a 'ta undo' subcommand

cli/src/invocation/cmd/mod.rs

@@ -8,4 +8,5 @@ pub(crate) mod info;
 pub(crate) mod modify;
 pub(crate) mod report;
 pub(crate) mod sync;
+pub(crate) mod undo;
 pub(crate) mod version;

cli/src/invocation/cmd/undo.rs

@@ -0,0 +1,28 @@
+use taskchampion::Replica;
+use termcolor::WriteColor;
+
+pub(crate) fn execute<W: WriteColor>(w: &mut W, replica: &mut Replica) -> Result<(), crate::Error> {
+    if replica.undo()? {
+        writeln!(w, "Undo successful.")?;
+    } else {
+        writeln!(w, "Nothing to undo.")?;
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::invocation::test::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn test_undo() {
+        let mut w = test_writer();
+        let mut replica = test_replica();
+
+        // Note that the details of the actual undo operation are tested thoroughly in the taskchampion crate
+        execute(&mut w, &mut replica).unwrap();
+        assert_eq!(&w.into_string(), "Nothing to undo.\n")
+    }
+}

cli/src/invocation/mod.rs

@@ -90,6 +90,13 @@ pub(crate) fn invoke(command: Command, settings: Settings) -> Result<(), crate::
             return cmd::sync::execute(&mut w, &mut replica, &settings, &mut server);
         }
 
+        Command {
+            subcommand: Subcommand::Undo,
+            ..
+        } => {
+            return cmd::undo::execute(&mut w, &mut replica);
+        }
+
         // handled in the first match, but here to ensure this match is exhaustive
         Command {
             subcommand: Subcommand::Help { .. },