add a 'task sync' command using a copy of the test server

cli/src/cmd/shared.rs

@@ -1,6 +1,7 @@
 use clap::Arg;
 use failure::{format_err, Fallible};
-use taskchampion::{Replica, Task, Uuid};
+use std::path::Path;
+use taskchampion::{server, taskstorage, Replica, Task, Uuid};
 
 pub(super) fn task_arg<'a>() -> Arg<'a, 'a> {
     Arg::with_name("task")
@@ -26,3 +27,31 @@ pub(super) fn get_task<S: AsRef<str>>(replica: &mut Replica, task_arg: S) -> Fal
 
     Err(format_err!("Cannot interpret {:?} as a task", task_arg))
 }
+
+/// A command invocation contains all of the necessary regarding a single invocation of the CLI.
+#[derive(Debug)]
+pub struct CommandInvocation {
+    pub(crate) subcommand: Box<dyn super::SubCommandInvocation>,
+}
+
+impl CommandInvocation {
+    pub(crate) fn new(subcommand: Box<dyn super::SubCommandInvocation>) -> Self {
+        Self { subcommand }
+    }
+
+    pub fn run(self) -> Fallible<()> {
+        self.subcommand.run(&self)
+    }
+
+    // -- utilities for command invocations
+
+    pub(super) fn get_replica(&self) -> Replica {
+        Replica::new(Box::new(
+            taskstorage::KVStorage::new(Path::new("/tmp/tasks")).unwrap(),
+        ))
+    }
+
+    pub(super) fn get_server(&self) -> impl server::Server {
+        server::LocalServer::new()
+    }
+}