commit: add a helper for returning parent tree of Commit

The pattern of getting the parent tree of a commit gets repeated a
bit. Let's add a helper on `Commit`.

cli/src/commands/commit.rs

@@ -14,7 +14,6 @@
 
 use jj_lib::object_id::ObjectId;
 use jj_lib::repo::Repo;
-use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
 use crate::cli_util::CommandHelper;
@@ -61,7 +60,7 @@ pub(crate) fn cmd_commit(
     let diff_selector =
         workspace_command.diff_selector(ui, args.tool.as_deref(), args.interactive)?;
     let mut tx = workspace_command.start_transaction();
-    let base_tree = merge_commit_trees(tx.repo(), &commit.parents())?;
+    let base_tree = commit.parent_tree(tx.repo())?;
     let instructions = format!(
         "\
 You are splitting the working-copy commit: {}

cli/src/commands/diff.rs

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
 use crate::cli_util::{print_unmatched_explicit_paths, CommandHelper, RevisionArg};
@@ -72,8 +71,7 @@ pub(crate) fn cmd_diff(
     } else {
         let commit = workspace_command
             .resolve_single_rev(args.revision.as_ref().unwrap_or(&RevisionArg::AT))?;
-        let parents = commit.parents();
-        from_tree = merge_commit_trees(workspace_command.repo().as_ref(), &parents)?;
+        from_tree = commit.parent_tree(workspace_command.repo().as_ref())?;
         to_tree = commit.tree()?
     }
     let fileset_expression = workspace_command.parse_file_patterns(&args.paths)?;

cli/src/commands/restore.rs

@@ -15,7 +15,7 @@
 use std::io::Write;
 
 use jj_lib::object_id::ObjectId;
-use jj_lib::rewrite::{merge_commit_trees, restore_tree};
+use jj_lib::rewrite::restore_tree;
 use tracing::instrument;
 
 use crate::cli_util::{CommandHelper, RevisionArg};
@@ -94,7 +94,7 @@ pub(crate) fn cmd_restore(
     } else {
         to_commit = workspace_command
             .resolve_single_rev(args.changes_in.as_ref().unwrap_or(&RevisionArg::AT))?;
-        from_tree = merge_commit_trees(workspace_command.repo().as_ref(), &to_commit.parents())?;
+        from_tree = to_commit.parent_tree(workspace_command.repo().as_ref())?;
     }
     workspace_command.check_rewritable([to_commit.id()])?;
 

cli/src/commands/split.rs

@@ -15,7 +15,6 @@ use std::io::Write;
 
 use jj_lib::object_id::ObjectId;
 use jj_lib::repo::Repo;
-use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
 use crate::cli_util::{CommandHelper, RevisionArg};
@@ -76,7 +75,7 @@ pub(crate) fn cmd_split(
     )?;
     let mut tx = workspace_command.start_transaction();
     let end_tree = commit.tree()?;
-    let base_tree = merge_commit_trees(tx.repo(), &commit.parents())?;
+    let base_tree = commit.parent_tree(tx.repo())?;
     let instructions = format!(
         "\
 You are splitting a commit into two: {}

cli/src/commands/squash.rs

@@ -19,7 +19,6 @@ use jj_lib::merged_tree::MergedTree;
 use jj_lib::object_id::ObjectId;
 use jj_lib::repo::Repo;
 use jj_lib::revset;
-use jj_lib::rewrite::merge_commit_trees;
 use jj_lib::settings::UserSettings;
 use tracing::instrument;
 
@@ -191,7 +190,7 @@ pub fn move_diff(
     }
     let mut source_commits = vec![];
     for source in sources {
-        let parent_tree = merge_commit_trees(tx.repo(), &source.parents())?;
+        let parent_tree = source.parent_tree(tx.repo())?;
         let source_tree = source.tree()?;
         let instructions = format!(
             "\

cli/src/commands/status.rs

@@ -15,7 +15,6 @@
 use itertools::Itertools;
 use jj_lib::repo::Repo;
 use jj_lib::revset::{RevsetExpression, RevsetFilterPredicate};
-use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
 use crate::cli_util::{print_conflicted_paths, CommandHelper};
@@ -59,7 +58,7 @@ pub(crate) fn cmd_status(
     let formatter = formatter.as_mut();
 
     if let Some(wc_commit) = &maybe_wc_commit {
-        let parent_tree = merge_commit_trees(repo.as_ref(), &wc_commit.parents())?;
+        let parent_tree = wc_commit.parent_tree(repo.as_ref())?;
         let tree = wc_commit.tree()?;
         if tree.id() == parent_tree.id() {
             writeln!(formatter, "The working copy is clean")?;

cli/src/commands/unsquash.rs

@@ -14,7 +14,6 @@
 
 use jj_lib::matchers::EverythingMatcher;
 use jj_lib::object_id::ObjectId;
-use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
 use crate::cli_util::{CommandHelper, RevisionArg};
@@ -69,7 +68,7 @@ pub(crate) fn cmd_unsquash(
         None
     };
     let mut tx = workspace_command.start_transaction();
-    let parent_base_tree = merge_commit_trees(tx.repo(), &parent.parents())?;
+    let parent_base_tree = parent.parent_tree(tx.repo())?;
     let new_parent_tree_id;
     if let Some(diff_editor) = &interactive_editor {
         let instructions = format!(

cli/src/commit_templater.rs

@@ -22,13 +22,13 @@ use itertools::Itertools as _;
 use jj_lib::backend::{ChangeId, CommitId};
 use jj_lib::commit::Commit;
 use jj_lib::extensions_map::ExtensionsMap;
+use jj_lib::git;
 use jj_lib::hex_util::to_reverse_hex;
 use jj_lib::id_prefix::IdPrefixContext;
 use jj_lib::object_id::ObjectId as _;
 use jj_lib::op_store::{RefTarget, WorkspaceId};
 use jj_lib::repo::Repo;
 use jj_lib::revset::{self, Revset, RevsetExpression, RevsetParseContext};
-use jj_lib::{git, rewrite};
 use once_cell::unsync::OnceCell;
 
 use crate::template_builder::{
@@ -649,7 +649,7 @@ fn builtin_commit_methods<'repo>() -> CommitTemplateBuildMethodFnMap<'repo, Comm
             if let [parent] = &commit.parents()[..] {
                 return Ok(parent.tree_id() == commit.tree_id());
             }
-            let parent_tree = rewrite::merge_commit_trees(repo, &commit.parents())?;
+            let parent_tree = commit.parent_tree(repo)?;
             Ok(*commit.tree_id() == parent_tree.id())
         });
         Ok(L::wrap_boolean(out_property))

lib/src/commit.rs

@@ -21,6 +21,8 @@ use std::sync::Arc;
 
 use crate::backend::{self, BackendResult, ChangeId, CommitId, MergedTreeId, Signature};
 use crate::merged_tree::MergedTree;
+use crate::repo::Repo;
+use crate::rewrite::merge_commit_trees;
 use crate::signing::{SignResult, Verification};
 use crate::store::Store;
 
@@ -108,6 +110,12 @@ impl Commit {
         &self.data.root_tree
     }
 
+    /// Return the parent tree, merging the parent trees if there are multiple
+    /// parents.
+    pub fn parent_tree(&self, repo: &dyn Repo) -> BackendResult<MergedTree> {
+        merge_commit_trees(repo, &self.parents())
+    }
+
     pub fn has_conflict(&self) -> BackendResult<bool> {
         if let MergedTreeId::Merge(tree_ids) = self.tree_id() {
             Ok(!tree_ids.is_resolved())

lib/src/rewrite.rs

@@ -306,8 +306,8 @@ pub fn rebase_to_dest_parent(
     if source.parent_ids() == destination.parent_ids() {
         Ok(source.tree()?)
     } else {
-        let destination_parent_tree = merge_commit_trees(repo, &destination.parents())?;
-        let source_parent_tree = merge_commit_trees(repo, &source.parents())?;
+        let destination_parent_tree = destination.parent_tree(repo)?;
+        let source_parent_tree = source.parent_tree(repo)?;
         let source_tree = source.tree()?;
         let rebased_tree = destination_parent_tree.merge(&source_parent_tree, &source_tree)?;
         Ok(rebased_tree)
@@ -320,7 +320,7 @@ pub fn back_out_commit(
     old_commit: &Commit,
     new_parents: &[Commit],
 ) -> BackendResult<Commit> {
-    let old_base_tree = merge_commit_trees(mut_repo, &old_commit.parents())?;
+    let old_base_tree = old_commit.parent_tree(mut_repo)?;
     let new_base_tree = merge_commit_trees(mut_repo, new_parents)?;
     let old_tree = old_commit.tree()?;
     let new_tree = new_base_tree.merge(&old_tree, &old_base_tree)?;