feat(revset): add + as shortcut for "only child"

eg `@+`, `abc123+`, `current(foo)+`, etc

Author: claytonrcarter

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] - ReleaseDate
 
+### Added
+
+- (#1461): Added `+` postfix revset operator; evaluates to "only child".
+
 ## [v0.10.0] - 2024-10-10
 
 ### Added

git-branchless-revset/src/grammar.lalrpop

@@ -9,6 +9,7 @@ grammar;
 pub Expr: Expr<'input> = {
     <lhs:Expr> "|"  <rhs:Expr2> => Expr::FunctionCall(Cow::Borrowed("union"), vec![lhs, rhs]),
     <lhs:Expr> "+"  <rhs:Expr2> => Expr::FunctionCall(Cow::Borrowed("union"), vec![lhs, rhs]),
+    <lhs:Expr> "+"              => Expr::FunctionCall(Cow::Borrowed("sole"),  vec![Expr::FunctionCall(Cow::Borrowed("children"), vec![lhs])]),
     <lhs:Expr> "or" <rhs:Expr2> => Expr::FunctionCall(Cow::Borrowed("union"), vec![lhs, rhs]),
     <Expr2>,
 }

git-branchless-revset/src/parser.rs

@@ -667,4 +667,28 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_revset_child_operator() -> eyre::Result<()> {
+        // TODO can we get a revset operator to complement ~/^?
+        insta::assert_debug_snapshot!(parse("foo+"), @r###"
+        Ok(
+            FunctionCall(
+                "sole",
+                [
+                    FunctionCall(
+                        "children",
+                        [
+                            Name(
+                                "foo",
+                            ),
+                        ],
+                    ),
+                ],
+            ),
+        )
+        "###);
+
+        Ok(())
+    }
 }