Add missing commondir accessor (#38998)

types/nodegit/nodegit-tests.ts

@@ -69,3 +69,4 @@ signature.email();
 signature.when();
 
 repo.createBlobFromBuffer(Buffer.from("test")).then((oid: Git.Oid) => oid.cpy());
+repo.commondir();

types/nodegit/repository.d.ts

@@ -47,6 +47,7 @@ export class Repository {
     static wrapOdb(odb: Odb): Promise<Repository>;
 
     cleanup(): void;
+    commondir(): string;
     config(): Promise<Config>;
     configSnapshot(): Promise<Config>;
     detachHead(): number;