use a generic Write instance for command output

cli/src/invocation/cmd/gc.rs

@@ -1,21 +1,23 @@
 use failure::Fallible;
 use taskchampion::Replica;
+use termcolor::WriteColor;
 
-pub(crate) fn execute(replica: &mut Replica) -> Fallible<()> {
+pub(crate) fn execute<W: WriteColor>(w: &mut W, replica: &mut Replica) -> Fallible<()> {
     replica.gc()?;
-    println!("garbage collected.");
+    write!(w, "garbage collected.\n")?;
     Ok(())
 }
 
 #[cfg(test)]
 mod test {
     use super::*;
-    use crate::invocation::cmd::test::test_replica;
+    use crate::invocation::cmd::test::*;
 
     #[test]
     fn test_gc() {
+        let mut w = test_writer();
         let mut replica = test_replica();
-        execute(&mut replica).unwrap();
-        // this mostly just needs to not fail!
+        execute(&mut w, &mut replica).unwrap();
+        assert_eq!(&w.into_string(), "garbage collected.\n")
     }
 }