templater: fast-path empty and conflict to not read trees

When there's a single parent, we can determine if a commit is empty by
just comparing the tree ids. Also, when using tree-level conflicts, we
don't need to read the trees to determine if there's a conflict. This
patch adds both of those fast paths, speeding up `jj log -r ::main`
from 317 ms to 227 ms (-28.4%). It has much larger impact with our
cloud-based backend at Google (~5x faster).

I made the same fix in the revset engine and the Git push code (thanks
to Yuya for the suggestion).

cli/src/commands/git.rs

@@ -905,7 +905,7 @@ fn cmd_git_push(
         {
             reasons.push("it has no author and/or committer set");
         }
-        if commit.tree()?.has_conflict() {
+        if commit.has_conflict()? {
             reasons.push("it has conflicts");
         }
         if !reasons.is_empty() {

cli/src/commit_templater.rs

@@ -299,10 +299,13 @@ fn build_commit_keyword_opt<'repo>(
             let maybe_entries = repo.resolve_change_id(commit.change_id());
             maybe_entries.map_or(true, |entries| !entries.contains(commit.id()))
         })),
-        "conflict" => language.wrap_boolean(wrap_fn(property, |commit| {
-            commit.tree().unwrap().has_conflict()
-        })),
+        "conflict" => {
+            language.wrap_boolean(wrap_fn(property, |commit| commit.has_conflict().unwrap()))
+        }
         "empty" => language.wrap_boolean(wrap_fn(property, |commit| {
+            if let [parent] = &commit.parents()[..] {
+                return parent.tree_id() == commit.tree_id();
+            }
             let parent_tree = rewrite::merge_commit_trees(repo, &commit.parents()).unwrap();
             *commit.tree_id() == parent_tree.id()
         })),

lib/src/commit.rs

@@ -108,6 +108,14 @@ impl Commit {
         &self.data.root_tree
     }
 
+    pub fn has_conflict(&self) -> Result<bool, BackendError> {
+        if let MergedTreeId::Merge(tree_ids) = self.tree_id() {
+            Ok(!tree_ids.is_resolved())
+        } else {
+            Ok(self.tree()?.has_conflict())
+        }
+    }
+
     pub fn change_id(&self) -> &ChangeId {
         &self.data.change_id
     }

lib/src/default_revset_engine.rs

@@ -869,7 +869,7 @@ fn build_predicate_fn<'index>(
         }
         RevsetFilterPredicate::HasConflict => pure_predicate_fn(move |entry| {
             let commit = store.get_commit(&entry.commit_id()).unwrap();
-            commit.tree().unwrap().has_conflict()
+            commit.has_conflict().unwrap()
         }),
     }
 }