Rename replica.gc to replica.rebuild_working_set

The command-line operation is still `gc`, but we'll break that down into
finer pieces in the replica.

cli/src/invocation/cmd/gc.rs

@@ -3,7 +3,8 @@ use taskchampion::Replica;
 use termcolor::WriteColor;
 
 pub(crate) fn execute<W: WriteColor>(w: &mut W, replica: &mut Replica) -> Fallible<()> {
-    replica.gc()?;
+    log::debug!("rebuilding working set");
+    replica.rebuild_working_set()?;
     writeln!(w, "garbage collected.")?;
     Ok(())
 }