rust-dirstatemap: add `clear_cached_mtime` helper method

This will help remove the `get_or_insert` method, which is dangerous because it
does not take the `DirstateMap` counters into account.

Differential Revision: https://phab.mercurial-scm.org/D12525

rust/hg-core/src/dirstate_tree/dirstate_map.rs

@@ -803,6 +803,26 b" impl<'on_disk> DirstateMap<'on_disk> {"
         Ok(())
     }
 
+    /// Clears the cached mtime for the (potential) folder at `path`.
+    pub(super) fn clear_cached_mtime(
+        &mut self,
+        path: &HgPath,
+    ) -> Result<(), DirstateV2ParseError> {
+        let node = match DirstateMap::get_node_mut(
+            self.on_disk,
+            &mut self.unreachable_bytes,
+            &mut self.root,
+            path,
+        )? {
+            Some(node) => node,
+            None => return Ok(()),
+        };
+        if let NodeData::CachedDirectory { .. } = &node.data {
+            node.data = NodeData::None
+        }
+        Ok(())
+    }
+
     fn iter_nodes<'tree>(
         &'tree self,
     ) -> impl Iterator<

rust/hg-core/src/dirstate_tree/status.rs

@@ -140,10 +140,7 b" pub fn status<'dirstate>("
     // Remove outdated mtimes before adding new mtimes, in case a given
     // directory is both
     for path in &outdated {
-        let node = dmap.get_or_insert(path)?;
-        if let NodeData::CachedDirectory { .. } = &node.data {
-            node.data = NodeData::None
-        }
+        dmap.clear_cached_mtime(path)?;
     }
     for (path, mtime) in &new_cachable {
         let node = dmap.get_or_insert(path)?;