Add a dry_run flag to SnapshotOptions.

This commit only adds the flag to the list of options, and does not provide an implementation. When set, it simply returns an error. 

At Google, we are implementing a tool that needs to know the state of the jj repo constantly. Always snapshotting and updating .jj/working_copy/checkout increases the likelihood of write races. To fix this, I'd like to add a --dry-run flag to SnapshotOptions to make it not update the working copy. This is similar to the request in jj-vcs#2562

Author: kevincliao

cli/src/cli_util.rs

@@ -1409,6 +1409,7 @@ to the current parents may contain changes from multiple commits.
             start_tracking_matcher,
             max_new_file_size,
             conflict_marker_style,
+            dry_run: false,
         })
     }
 

cli/src/merge_tools/diff_working_copies.rs

@@ -299,6 +299,7 @@ diff editing in mind and be a little inaccurate.
             start_tracking_matcher: &EverythingMatcher,
             max_new_file_size: u64::MAX,
             conflict_marker_style,
+            dry_run: false,
         })?;
         Ok(output_tree_state.current_tree_id().clone())
     }

lib/src/local_working_copy.rs

@@ -966,8 +966,16 @@ impl TreeState {
             start_tracking_matcher,
             max_new_file_size,
             conflict_marker_style,
+            dry_run,
         } = options;
 
+        if dry_run {
+            return Err(SnapshotError::Other {
+                message: "dry_run is not supported yet".to_string(),
+                err: "dry_run is not supported yet".into(),
+            });
+        }
+
         let sparse_matcher = self.sparse_matcher();
 
         let fsmonitor_clock_needs_save = *fsmonitor_settings != FsmonitorSettings::None;

lib/src/working_copy.rs

@@ -222,6 +222,9 @@ pub struct SnapshotOptions<'a> {
     pub max_new_file_size: u64,
     /// Expected conflict marker style for checking for changed files.
     pub conflict_marker_style: ConflictMarkerStyle,
+    /// If true, skip any updates to the working copy metadata files when
+    /// snapshotting.
+    pub dry_run: bool,
 }
 
 impl SnapshotOptions<'_> {
@@ -234,6 +237,7 @@ impl SnapshotOptions<'_> {
             start_tracking_matcher: &EverythingMatcher,
             max_new_file_size: u64::MAX,
             conflict_marker_style: ConflictMarkerStyle::default(),
+            dry_run: false,
         }
     }
 }

lib/tests/test_local_working_copy.rs

@@ -2222,3 +2222,15 @@ fn test_snapshot_max_new_file_size() {
         UntrackedReason::FileTooLarge { .. }
     );
 }
+
+#[test]
+fn test_dry_run() {
+    let mut test_workspace = TestWorkspace::init();
+    let options = SnapshotOptions {
+        dry_run: true,
+        ..SnapshotOptions::empty_for_test()
+    };
+    test_workspace
+        .snapshot_with_options(&options)
+        .expect_err("dry_run is not implemented yet");
+}