revset: remove redundant candidates argument from merges()

Since 'merges()' just filters the candidates set per item, it doesn't need
a candidates argument. Perhaps, 'merges(x)' could be a predicate to select
merge commits within a subgraph 'x', but I don't know if that would be
useful.

CHANGELOG.md

@@ -10,7 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Breaking changes
 
 * Dropped candidates set argument from `description(needle)`, `author(needle)`,
-  `committer(needle)` revsets. Use `x & description(needle)` instead.
+  `committer(needle)`, `merges()` revsets. Use `x & description(needle)`
+  instead.
 
 ### New features
 

docs/revsets.md

@@ -101,8 +101,7 @@ revsets (expressions) as arguments.
   If `x` was not specified, it selects all visible heads (as if you had said
   `heads(all())`).
 * `roots(x)`: Commits in `x` that are not descendants of other commits in `x`.
-* `merges([x])`: Merge commits within `x`. If `x` was not specified, it selects
-  all visible merge commits (as if you had said `merges(all())`).
+* `merges()`: Merge commits.
 * `description(needle)`: Commits with the given string in their
   description.
 * `author(needle)`: Commits with the given string in the author's name or

lib/src/revset.rs

@@ -745,18 +745,14 @@ fn parse_function_expression(
             }
         }
         "merges" => {
-            if arg_count > 1 {
-                return Err(RevsetParseError::InvalidFunctionArguments {
-                    name,
-                    message: "Expected 0 or 1 arguments".to_string(),
-                });
-            }
-            let candidates = if arg_count == 0 {
-                RevsetExpression::all()
+            if arg_count == 0 {
+                Ok(RevsetExpression::all().with_parent_count(2..u32::MAX))
             } else {
-                parse_expression_rule(argument_pairs.next().unwrap().into_inner())?
-            };
-            Ok(candidates.with_parent_count(2..u32::MAX))
+                Err(RevsetParseError::InvalidFunctionArguments {
+                    name,
+                    message: "Expected 0 arguments".to_string(),
+                })
+            }
         }
         "description" | "author" | "committer" | "file" => {
             if arg_count != 1 {

lib/tests/test_revset.rs

@@ -1388,7 +1388,7 @@ fn test_evaluate_expression_merges(use_git: bool) {
     assert_eq!(
         resolve_commit_ids(
             mut_repo.as_repo_ref(),
-            &format!("merges(:{})", commit5.id().hex())
+            &format!(":{} & merges()", commit5.id().hex())
         ),
         vec![commit5.id().clone()]
     );