cli: diff: add fileset-alias option

cli/src/commands/diff.rs

@@ -56,6 +56,9 @@ pub(crate) struct DiffArgs {
     /// Show changes to this revision
     #[arg(long, short, conflicts_with = "revision", add = ArgValueCandidates::new(complete::all_revisions))]
     to: Option<RevisionArg>,
+    /// Restrict the diff to the paths specified in [fileset-aliases.{}]
+    #[arg(long, short = 'P', conflicts_with = "paths")]
+    fileset_alias: Option<String>,
     /// Restrict the diff to these paths
     #[arg(value_hint = clap::ValueHint::AnyPath)]
     paths: Vec<String>,
@@ -71,7 +74,14 @@ pub(crate) fn cmd_diff(
 ) -> Result<(), CommandError> {
     let workspace_command = command.workspace_helper(ui)?;
     let repo = workspace_command.repo();
-    let fileset_expression = workspace_command.parse_file_patterns(ui, &args.paths)?;
+    let fileset_expression = if let Some(alias) = &args.fileset_alias {
+        let fileset = workspace_command
+            .settings()
+            .get_string(&format!("fileset-aliases.{}", alias))?;
+        workspace_command.parse_file_patterns(ui, &[fileset])?
+    } else {
+        workspace_command.parse_file_patterns(ui, &args.paths)?
+    };
     let matcher = fileset_expression.to_matcher();
     let resolve_revision = |r: &Option<RevisionArg>| {
         workspace_command.resolve_single_rev(ui, r.as_ref().unwrap_or(&RevisionArg::AT))