message: Use `getOrCreate` helper in editManager.ts
  This simplifies a helper function by using a common helper function
diff: @@ -8,7 +8,7 @@ import { ChangeFamily } from "../change-family";
  import { TaggedChange, RevisionTag, tagChange, tagInverse } from "../rebase";
  import { SimpleDependee } from "../dependency-tracking";
  import { AnchorSet, Delta, emptyDelta } from "../tree";
  -import { brand, Brand, fail, RecursiveReadonly } from "../../util";
  +import { brand, Brand, fail, getOrCreate, RecursiveReadonly } from "../../util";
  export interface Commit<TChangeset> {
  sessionId: SessionId;
  @@ -378,10 +378,7 @@ export class EditManager<
  }
  private getOrCreateBranch(sessionId: SessionId, refSeq: SeqNumber): Branch<TChangeset> {
  - if (!this.branches.has(sessionId)) {
  - this.branches.set(sessionId, { localChanges: [], refSeq });
  - }
  - return this.branches.get(sessionId) as Branch<TChangeset>;
  + return getOrCreate(this.branches, sessionId, () => ({ localChanges: [], refSeq }));
  }
  private inverseFromCommit(commit: Commit<TChangeset>): TaggedChange<TChangeset> {
  