POC: pixi diff

Example:

```
pixi diff --old-lockfile <(git show HEAD~100:pixi.lock)
```

src/cli/diff.rs

@@ -0,0 +1,47 @@
+use std::path::{Path, PathBuf};
+
+use clap::Parser;
+use miette::IntoDiagnostic;
+use pixi_config::{self, ConfigCli};
+use rattler_lock::LockFile;
+
+use crate::{
+    cli::update::{LockFileDiff, LockFileJsonDiff},
+    Project,
+};
+
+use super::cli_config::ProjectConfig;
+
+#[derive(Parser, Debug, Default)]
+pub struct Args {
+    #[clap(flatten)]
+    pub config: ConfigCli,
+
+    #[clap(flatten)]
+    pub project_config: ProjectConfig,
+
+    #[arg(long)]
+    pub old_lockfile: PathBuf,
+}
+
+pub async fn execute(args: Args) -> miette::Result<()> {
+    let lock = LockFile::from_path(Path::new("pixi.lock")).into_diagnostic()?;
+
+    let input: Box<dyn std::io::Read + 'static> = if args.old_lockfile.as_os_str() == "-" {
+        Box::new(std::io::stdin())
+    } else {
+        Box::new(std::fs::File::open(&args.old_lockfile).into_diagnostic()?)
+    };
+
+    let old = LockFile::from_reader(input).into_diagnostic()?;
+
+    let diff = LockFileDiff::from_lock_files(&old, &lock);
+    let config = args.config;
+    let project = Project::load_or_else_discover(args.project_config.manifest_path.as_deref())?
+        .with_cli_config(config);
+    let json_diff = LockFileJsonDiff::new(&project, diff);
+    let json = serde_json::to_string_pretty(&json_diff).expect("failed to convert to json");
+    println!("{}", json);
+
+    Ok(())
+}

src/cli/mod.rs

@@ -18,6 +18,7 @@ pub mod clean;
 pub mod cli_config;
 pub mod completion;
 pub mod config;
+pub mod diff;
 mod exec;
 pub mod global;
 pub mod has_specs;
@@ -104,6 +105,7 @@ pub enum Command {
     #[clap(visible_alias = "s")]
     Shell(shell::Args),
     ShellHook(shell_hook::Args),
+    Diff(diff::Args),
 
     // Project modification commands
     Project(project::Args),
@@ -281,6 +283,7 @@ pub async fn execute_command(command: Command) -> miette::Result<()> {
         Command::Tree(cmd) => tree::execute(cmd).await,
         Command::Update(cmd) => update::execute(cmd).await,
         Command::Exec(args) => exec::execute(args).await,
+        Command::Diff(args) => diff::execute(args).await,
     }
 }
 

src/cli/update.rs

@@ -638,7 +638,7 @@ pub struct LockFileJsonDiff {
 }
 
 impl LockFileJsonDiff {
-    fn new(project: &Project, value: LockFileDiff) -> Self {
+    pub fn new(project: &Project, value: LockFileDiff) -> Self {
         let mut environment = IndexMap::new();
 
         for (environment_name, environment_diff) in value.environment {