revset: ad-hoc optimization for range queries containing unwanted wan…

…ted heads

In my linux stable mirror, this makes the default log revset evaluation super
fast. immutable_heads(), if configured properly, includes many historical
branch heads which are also the visible heads.

revsets/immutable_heads()..
---------------------------
0     12.27     117.1±0.77m
3      1.00       9.5±0.08m

cli/testing/bench-revsets-git.txt

@@ -16,6 +16,8 @@ v2.39.0::v2.40.0
 # Tags and branches
 tags()
 branches()
+# Local changes
+(tags() | remote_branches())..
 # Intersection of range with a small subset
 tags() & ::v2.40.0
 v2.39.0 & ::v2.40.0

lib/src/default_index/revset_engine.rs

@@ -764,8 +764,16 @@ impl<'index> EvaluationContext<'index> {
             } => {
                 let root_set = self.evaluate(roots)?;
                 let root_positions = root_set.positions(index).collect_vec();
+                // Pre-filter heads so queries like 'immutable_heads()..' can
+                // terminate early. immutable_heads() usually includes some
+                // visible heads, which can be trivially rejected.
                 let head_set = self.evaluate(heads)?;
-                let head_positions = head_set.positions(index).collect_vec();
+                let head_positions = difference_by(
+                    head_set.positions(index),
+                    root_positions.iter().copied(),
+                    |pos1, pos2| pos1.cmp(pos2).reverse(),
+                )
+                .collect_vec();
                 if generation == &GENERATION_RANGE_FULL {
                     Ok(Box::new(RevWalkRevset::new(move |index| {
                         Box::new(index.walk_revs(&head_positions, &root_positions))