template: add "empty" template item

CHANGELOG.md

@@ -46,6 +46,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   e.g. `color.error = { bg = "red", bold = true, underline = true }` in your
   `~/.jjconfig.toml`.
 
+* The `empty` condition in templates is true when the commit makes no change to
+  the three compared to its parents.
+
 ### Fixed bugs
 
 * When sharing the working copy with a Git repo, we used to forget to export

src/template_parser.rs

@@ -25,7 +25,7 @@ use crate::formatter::PlainTextFormatter;
 use crate::templater::{
     AuthorProperty, BranchProperty, ChangeIdProperty, CommitIdKeyword, CommitterProperty,
     ConditionalTemplate, ConflictProperty, ConstantTemplateProperty, DescriptionProperty,
-    DivergentProperty, DynamicLabelTemplate, GitRefsProperty, IsGitHeadProperty,
+    DivergentProperty, DynamicLabelTemplate, EmptyProperty, GitRefsProperty, IsGitHeadProperty,
     IsWorkingCopyProperty, LabelTemplate, ListTemplate, LiteralTemplate, SignatureTimestamp,
     StringPropertyTemplate, TagProperty, Template, TemplateFunction, TemplateProperty,
     WorkingCopiesProperty,
@@ -316,6 +316,7 @@ fn parse_commit_keyword<'a>(
         "is_git_head" => Property::Boolean(Box::new(IsGitHeadProperty::new(repo))),
         "divergent" => Property::Boolean(Box::new(DivergentProperty::new(repo))),
         "conflict" => Property::Boolean(Box::new(ConflictProperty)),
+        "empty" => Property::Boolean(Box::new(EmptyProperty { repo })),
         name => panic!("unexpected identifier: {name}"),
     };
     PropertyAndLabels(property, vec![pair.as_str().to_string()])

src/templater.rs

@@ -23,6 +23,7 @@ use jujutsu_lib::commit::Commit;
 use jujutsu_lib::op_store::WorkspaceId;
 use jujutsu_lib::repo::RepoRef;
 use jujutsu_lib::revset::RevsetExpression;
+use jujutsu_lib::rewrite::merge_commit_trees;
 
 use crate::formatter::Formatter;
 
@@ -440,3 +441,13 @@ impl TemplateProperty<Signature, Timestamp> for SignatureTimestamp {
         context.timestamp.clone()
     }
 }
+
+pub struct EmptyProperty<'a> {
+    pub repo: RepoRef<'a>,
+}
+
+impl TemplateProperty<Commit, bool> for EmptyProperty<'_> {
+    fn extract(&self, context: &Commit) -> bool {
+        context.tree().id() == merge_commit_trees(self.repo, &context.parents()).id()
+    }
+}