templater: add special "self" variable to refer to top-level object

This allows us to call alias function with the top-level object.

For convenience, all self.<method>()s are available as keywords. I don't think
we'll want to deprecate them. It would be tedious if we had to specify
-T'self.commit_id()' instead of -Tcommit_id.

CHANGELOG.md

@@ -37,6 +37,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * Templates now support logical operators: `||`, `&&`, `!`
 
+* Templates now support the `self` keyword, which is the current commit in `jj
+  log`/`obslog` templates.
+
 * `jj show` now accepts `-T`/`--template` option to render its output using
   template
 

cli/src/template_builder.rs

@@ -866,6 +866,9 @@ pub fn build_expression<'a, L: TemplateLanguage<'a>>(
             if let Some(make) = build_ctx.local_variables.get(name) {
                 // Don't label a local variable with its name
                 Ok(Expression::unlabeled(make()))
+            } else if *name == "self" {
+                // "self" is a special variable, so don't label it
+                Ok(Expression::unlabeled(language.build_self()))
             } else {
                 build_keyword(language, build_ctx, name, node.span)
             }
@@ -1315,6 +1318,24 @@ mod tests {
         "###);
     }
 
+    #[test]
+    fn test_self_keyword() {
+        let mut env = TestTemplateEnv::default();
+        env.add_keyword("say_hello", |language| {
+            language.wrap_string(Literal("Hello".to_owned()))
+        });
+
+        insta::assert_snapshot!(env.render_ok(r#"self.say_hello()"#), @"Hello");
+        insta::assert_snapshot!(env.parse_err(r#"self"#), @r###"
+         --> 1:1
+          |
+        1 | self
+          | ^--^
+          |
+          = Expected expression of type "Template"
+        "###);
+    }
+
     #[test]
     fn test_boolean_cast() {
         let mut env = TestTemplateEnv::default();

docs/templates.md

@@ -9,17 +9,20 @@ A couple of `jj` commands accept a template via `-T`/`--template` option.
 ## Keywords
 
 Keywords represent objects of different types; the types are described in
-a follow-up section.
+a follow-up section. In addition to context-specific keywords, the top-level
+object can be referenced as `self`.
 
 ### Commit keywords
 
 In `jj log`/`jj obslog` templates, all 0-argument methods of [the `Commit`
-type](#commit-type) are available as keywords.
+type](#commit-type) are available as keywords. For example, `commit_id` is
+equivalent to `self.commit_id()`.
 
 ### Operation keywords
 
 In `jj op log` templates, all 0-argument methods of [the `Operation`
-type](#operation-type) are available as keywords.
+type](#operation-type) are available as keywords. For example,
+`current_operation` is equivalent to `self.current_operation()`.
 
 ## Operators