cli: extract new cli_util module

This commit moves much of the `commands` module that isn't specific to
a particular command into a new `cli_util`.

Much of this is actually not even CLI-specific, so we should move that
further down into the library, but that can come later. That includes
the code in `WorkspaceCommandHelper` for snapshotting the working copy
and automatically importing/exporting to a colocated Git working copy.

src/lib.rs

@@ -14,6 +14,7 @@
 
 #![deny(unused_must_use)]
 
+pub mod cli_util;
 pub mod commands;
 pub mod config;
 pub mod diff_edit;

src/main.rs

@@ -12,7 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use jujutsu::commands::{dispatch, CommandError};
+use jujutsu::cli_util::CommandError;
+use jujutsu::commands::dispatch;
 use jujutsu::config::read_config;
 use jujutsu::ui::Ui;
 use jujutsu_lib::settings::UserSettings;