git_backend: test conversion of parents, including root

We didn't seem to have any tests showing how we convert the set of
parents, and especially how we handle the root commit, so let's add
some.

lib/src/git_backend.rs

@@ -709,6 +709,56 @@ mod tests {
         );
     }
 
+    /// Test that parents get written correctly
+    #[test]
+    fn git_commit_parents() {
+        let temp_dir = testutils::new_temp_dir();
+        let store_path = temp_dir.path();
+        let git_repo_path = temp_dir.path().join("git");
+        let git_repo = git2::Repository::init(&git_repo_path).unwrap();
+
+        let backend = GitBackend::init_external(store_path, &git_repo_path);
+        let mut commit = Commit {
+            parents: vec![],
+            predecessors: vec![],
+            root_tree: backend.empty_tree_id().clone(),
+            change_id: ChangeId::from_hex("abc123"),
+            description: "".to_string(),
+            author: create_signature(),
+            committer: create_signature(),
+        };
+
+        // Only root commit as parent
+        commit.parents = vec![backend.root_commit_id().clone()];
+        let first_id = backend.write_commit(&commit).unwrap();
+        let first_commit = backend.read_commit(&first_id).unwrap();
+        assert_eq!(first_commit, commit);
+        let first_git_commit = git_repo.find_commit(git_id(&first_id)).unwrap();
+        assert_eq!(first_git_commit.parent_ids().collect_vec(), vec![]);
+
+        // Only non-root commit as parent
+        commit.parents = vec![first_id.clone()];
+        let second_id = backend.write_commit(&commit).unwrap();
+        let second_commit = backend.read_commit(&second_id).unwrap();
+        assert_eq!(second_commit, commit);
+        let second_git_commit = git_repo.find_commit(git_id(&second_id)).unwrap();
+        assert_eq!(
+            second_git_commit.parent_ids().collect_vec(),
+            vec![git_id(&first_id)]
+        );
+
+        // Merge commit
+        commit.parents = vec![first_id.clone(), second_id.clone()];
+        let merge_id = backend.write_commit(&commit).unwrap();
+        let merge_commit = backend.read_commit(&merge_id).unwrap();
+        assert_eq!(merge_commit, commit);
+        let merge_git_commit = git_repo.find_commit(git_id(&merge_id)).unwrap();
+        assert_eq!(
+            merge_git_commit.parent_ids().collect_vec(),
+            vec![git_id(&first_id), git_id(&second_id)]
+        );
+    }
+
     #[test]
     fn commit_has_ref() {
         let temp_dir = testutils::new_temp_dir();
@@ -738,32 +788,21 @@ mod tests {
             .unwrap()
             .map(|git_ref| git_ref.unwrap().target().unwrap())
             .collect_vec();
-        assert_eq!(
-            git_refs,
-            vec![Oid::from_bytes(commit_id.as_bytes()).unwrap()]
-        );
+        assert_eq!(git_refs, vec![git_id(&commit_id)]);
     }
 
     #[test]
     fn overlapping_git_commit_id() {
         let temp_dir = testutils::new_temp_dir();
         let store = GitBackend::init_internal(temp_dir.path());
-        let signature = Signature {
-            name: "Someone".to_string(),
-            email: "someone@example.com".to_string(),
-            timestamp: Timestamp {
-                timestamp: MillisSinceEpoch(0),
-                tz_offset: 0,
-            },
-        };
         let commit1 = Commit {
             parents: vec![],
             predecessors: vec![],
             root_tree: store.empty_tree_id().clone(),
             change_id: ChangeId::new(vec![]),
             description: "initial".to_string(),
-            author: signature.clone(),
-            committer: signature,
+            author: create_signature(),
+            committer: create_signature(),
         };
         let commit_id1 = store.write_commit(&commit1).unwrap();
         let mut commit2 = commit1;
@@ -772,4 +811,19 @@ mod tests {
         // committer timestamp of the commit it actually writes.
         assert_ne!(store.write_commit(&commit2).unwrap(), commit_id1);
     }
+
+    fn git_id(commit_id: &CommitId) -> Oid {
+        Oid::from_bytes(commit_id.as_bytes()).unwrap()
+    }
+
+    fn create_signature() -> Signature {
+        Signature {
+            name: "Someone".to_string(),
+            email: "someone@example.com".to_string(),
+            timestamp: Timestamp {
+                timestamp: MillisSinceEpoch(0),
+                tz_offset: 0,
+            },
+        }
+    }
 }