revsets: add git_head() revset

This is part of #44.

lib/src/revset.rs

@@ -202,6 +202,7 @@ pub enum RevsetExpression {
     RemoteBranches,
     Tags,
     GitRefs,
+    GitHead,
     ParentCount {
         candidates: Rc<RevsetExpression>,
         parent_count_range: Range<u32>,
@@ -260,6 +261,10 @@ impl RevsetExpression {
         Rc::new(RevsetExpression::GitRefs)
     }
 
+    pub fn git_head() -> Rc<RevsetExpression> {
+        Rc::new(RevsetExpression::GitHead)
+    }
+
     /// Commits in `self` that don't have descendants in `self`.
     // TODO: Perhaps this should be renamed to just `heads()` and the current
     // `heads()` should become `visible_heads()` or `current_heads()`.
@@ -644,6 +649,16 @@ fn parse_function_expression(
                 })
             }
         }
+        "git_head" => {
+            if arg_count == 0 {
+                Ok(RevsetExpression::git_head())
+            } else {
+                Err(RevsetParseError::InvalidFunctionArguments {
+                    name,
+                    message: "Expected 0 arguments".to_string(),
+                })
+            }
+        }
         "merges" => {
             if arg_count > 1 {
                 return Err(RevsetParseError::InvalidFunctionArguments {
@@ -1156,6 +1171,10 @@ pub fn evaluate_expression<'repo>(
             }
             Ok(revset_for_commit_ids(repo, &commit_ids))
         }
+        RevsetExpression::GitHead => {
+            let commit_ids = repo.view().git_head().into_iter().collect_vec();
+            Ok(revset_for_commit_ids(repo, &commit_ids))
+        }
         RevsetExpression::Description { needle, candidates } => {
             let candidates = candidates.evaluate(repo)?;
             let repo = repo;

lib/tests/test_revset.rs

@@ -980,6 +980,30 @@ fn test_evaluate_expression_git_refs(use_git: bool) {
     );
 }
 
+#[test_case(false ; "local backend")]
+#[test_case(true ; "git backend")]
+fn test_evaluate_expression_git_head(use_git: bool) {
+    let settings = testutils::user_settings();
+    let test_workspace = testutils::init_repo(&settings, use_git);
+    let repo = &test_workspace.repo;
+
+    let mut tx = repo.start_transaction("test");
+    let mut_repo = tx.mut_repo();
+
+    let commit1 = testutils::create_random_commit(&settings, repo).write_to_repo(mut_repo);
+
+    // Can get git head when it's not set
+    assert_eq!(
+        resolve_commit_ids(mut_repo.as_repo_ref(), "git_head()"),
+        vec![]
+    );
+    mut_repo.set_git_head(commit1.id().clone());
+    assert_eq!(
+        resolve_commit_ids(mut_repo.as_repo_ref(), "git_head()"),
+        vec![commit1.id().clone()]
+    );
+}
+
 #[test_case(false ; "local backend")]
 #[test_case(true ; "git backend")]
 fn test_evaluate_expression_branches(use_git: bool) {