templater: make "git_head" return boolean, render it as "git_head()"

For the same reason as the root revision is indicated by "root()".

CHANGELOG.md

@@ -16,7 +16,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * Evaluation error of `revsets.short-prefixes` configuration is now reported.
 
 * The `HEAD@git` symbol no longer resolves to the Git HEAD revision. Use
-  `git_head()` or `@-` revset expression instead.
+  `git_head()` or `@-` revset expression instead. The `git_head` template
+  keyword now returns a boolean.
 
 * Help command doesn't work recursively anymore, i.e. `jj workspace help root`
   doesn't work anymore.

cli/src/commit_templater.rs

@@ -30,7 +30,6 @@ use jj_lib::extensions_map::ExtensionsMap;
 use jj_lib::fileset;
 use jj_lib::fileset::FilesetDiagnostics;
 use jj_lib::fileset::FilesetExpression;
-use jj_lib::git;
 use jj_lib::id_prefix::IdPrefixContext;
 use jj_lib::id_prefix::IdPrefixIndex;
 use jj_lib::matchers::Matcher;
@@ -703,8 +702,11 @@ fn builtin_commit_methods<'repo>() -> CommitTemplateBuildMethodFnMap<'repo, Comm
         |language, _diagnostics, _build_ctx, self_property, function| {
             function.expect_no_arguments()?;
             let repo = language.repo;
-            let out_property = self_property.map(|commit| extract_git_head(repo, &commit));
-            Ok(L::wrap_ref_name_opt(out_property))
+            let out_property = self_property.map(|commit| {
+                let target = repo.view().git_head();
+                target.added_ids().contains(commit.id())
+            });
+            Ok(L::wrap_boolean(out_property))
         },
     );
     map.insert(
@@ -1230,14 +1232,6 @@ fn build_ref_names_index<'a>(
     index
 }
 
-fn extract_git_head(repo: &dyn Repo, commit: &Commit) -> Option<Rc<RefName>> {
-    let target = repo.view().git_head();
-    target
-        .added_ids()
-        .contains(commit.id())
-        .then(|| RefName::remote_only("HEAD", git::REMOTE_NAME_FOR_LOCAL_GIT_REPO, target.clone()))
-}
-
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum CommitOrChangeId {
     Commit(CommitId),

cli/src/config/templates.toml

@@ -78,7 +78,7 @@ if(root,
         bookmarks,
         tags,
         working_copies,
-        git_head,
+        if(git_head, label("git_head", "git_head()")),
         format_short_commit_id(commit_id),
         if(conflict, label("conflict", "conflict")),
         if(empty, label("empty", "(empty)")),
@@ -103,7 +103,7 @@ if(root,
         bookmarks,
         tags,
         working_copies,
-        git_head,
+        if(git_head, label("git_head", "git_head()")),
         format_short_commit_id(commit_id),
         if(conflict, label("conflict", "conflict")),
       ) ++ "\n",

cli/tests/test_commit_template.rs

@@ -671,27 +671,21 @@ fn test_log_git_head() {
     test_env.jj_cmd_ok(&repo_path, &["new", "-m=initial"]);
     std::fs::write(repo_path.join("file"), "foo\n").unwrap();
 
-    let template = r#"
-    separate(", ",
-      if(git_head, "name: " ++ git_head.name()),
-      "remote: " ++ git_head.remote(),
-    ) ++ "\n"
-    "#;
-    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", template]);
-    insta::assert_snapshot!(stdout, @r###"
-    @  remote: <Error: No RefName available>
-    ○  name: HEAD, remote: git
-    ◆  remote: <Error: No RefName available>
-    "###);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "git_head"]);
+    insta::assert_snapshot!(stdout, @r#"
+    @  false
+    ○  true
+    ◆  false
+    "#);
 
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "--color=always"]);
-    insta::assert_snapshot!(stdout, @r###"
+    insta::assert_snapshot!(stdout, @r#"
     @  rlvkpnrz [38;5;[email protected] 2001-02-03 08:05:09 50aaf475
     │  initial
-    ○  [1m[38;5;5mq[0m[38;5;8mpvuntsm[39m [38;5;[email protected][39m [38;5;6m2001-02-03 08:05:07[39m [38;5;2mHEAD@git[39m [1m[38;5;4m2[0m[38;5;8m30dd059[39m
+    ○  [1m[38;5;5mq[0m[38;5;8mpvuntsm[39m [38;5;[email protected][39m [38;5;6m2001-02-03 08:05:07[39m [38;5;2mgit_head()[39m [1m[38;5;4m2[0m[38;5;8m30dd059[39m
     │  (empty) (no description set)
     ◆  zzzzzzzz root() 00000000
-    "###);
+    "#);
 }
 
 #[test]