Only load first parent on commit diff

When generating diffs, the first parent should be used to show the
changes that were introduced in the change. Diffing against the other
parents slows down diff loading and does not provide any useful
information.

Author: MitMaro

src/git/commit_diff_loader.rs

@@ -30,34 +30,26 @@ impl<'options> CommitDiffLoader<'options> {
 		Self { config, repo }
 	}
 
-	pub(crate) fn load_from_hash(&self, oid: Oid) -> Result<Vec<CommitDiff>, git2::Error> {
+	pub(crate) fn load_from_hash(&self, oid: Oid) -> Result<CommitDiff, git2::Error> {
 		let repo = self.repo.lock();
 		let commit = repo.find_commit(oid)?;
-		let no_parents = commit.parent_ids().count() == 0;
+		// only the first parent matter for the diff, the second parent, if it exists, was only used
+		// for conflict resolution
+		let parent = commit.parents().next();
 
-		// some commits do not have parents, and can't have file stats
-		let diffs = if no_parents {
-			vec![self.load_diff(&repo, None, &commit)?]
-		}
-		else {
-			//
-			let mut diffs = vec![];
-			for parent in commit.parents() {
-				diffs.push(self.load_diff(&repo, Some(&parent), &commit)?);
-			}
-			diffs
-		};
-		Ok(diffs)
+		self.load_diff(&repo, parent, &commit)
 	}
 
 	#[expect(clippy::as_conversions, reason = "Mostly safe difference between APIs.")]
 	#[expect(clippy::unwrap_in_result, reason = "Unwrap usage failure considered a bug.")]
 	fn load_diff(
 		&self,
 		repo: &MutexGuard<'_, Repository>,
-		parent: Option<&git2::Commit<'_>>,
+		parent: Option<git2::Commit<'_>>,
 		commit: &git2::Commit<'_>,
 	) -> Result<CommitDiff, git2::Error> {
+		let parent_commit = parent.as_ref().map(Commit::from);
+
 		let mut diff_options = DiffOptions::new();
 		// include_unmodified added to find copies from unmodified files
 		_ = diff_options
@@ -148,7 +140,7 @@ impl<'options> CommitDiffLoader<'options> {
 
 		Ok(CommitDiff::new(
 			Commit::from(commit),
-			parent.map(Commit::from),
+			parent_commit,
 			fsb.build(),
 			number_files_changed,
 			number_insertions,
@@ -316,7 +308,7 @@ mod tests {
 	fn diff_from_head(repository: &crate::git::Repository, options: &CommitDiffLoaderOptions) -> CommitDiff {
 		let id = repository.commit_id_from_ref("refs/heads/main").unwrap();
 		let loader = CommitDiffLoader::new(repository.repository(), options);
-		loader.load_from_hash(id).unwrap().remove(0)
+		loader.load_from_hash(id).unwrap()
 	}
 
 	#[test]

src/git/repository.rs

@@ -60,11 +60,10 @@ impl Repository {
 			.id();
 		let diff_loader_repository = Arc::clone(&self.repository);
 		let loader = CommitDiffLoader::new(diff_loader_repository, config);
-		// TODO this is ugly because it assumes one parent
-		Ok(loader
+
+		loader
 			.load_from_hash(oid)
-			.map_err(|e| GitError::CommitLoad { cause: e })?
-			.remove(0))
+			.map_err(|e| GitError::CommitLoad { cause: e })
 	}
 }