mirror: make a data interface (#1291)

This is a quick fixup so that the coming createGraph module can be
properly tested.

Shout out to @Beanow for anticipating this need in a [review comment].

[review comment]: https://github.com/sourcecred/sourcecred/pull/1266#discussion_r314305108

Test plan: trivial refactor, run `yarn test`

src/plugins/discourse/mirror.js

@@ -15,6 +15,38 @@ import {
 // so that the cache will be properly invalidated.
 const VERSION = "discourse_mirror_v1";
 
+/**
+ * An interface for retrieving all of the Discourse data at once.
+ *
+ * Also has some convenience methods for interpeting the data (e.g. getting
+ * a post by its index in a topic).
+ *
+ * The mirror implements this; it's factored out as an interface for
+ * ease of testing.
+ */
+export interface DiscourseData {
+  /**
+   * Retrieve every Topic available.
+   *
+   * The order is unspecified.
+   */
+  topics(): $ReadOnlyArray<Topic>;
+
+  /**
+   * Retrieve every Post available.
+   *
+   * The order is unspecified.
+   */
+  posts(): $ReadOnlyArray<Post>;
+
+  /**
+   * Given a TopicId and a post number, find that numbered post within the topic.
+   *
+   * Returns undefined if no such post is available.
+   */
+  findPostInTopic(topicId: TopicId, indexWithinTopic: number): ?PostId;
+}
+
 /**
  * Mirrors data from the Discourse API into a local sqlite db.
  *
@@ -32,7 +64,7 @@ const VERSION = "discourse_mirror_v1";
  * Each Mirror instance is tied to a particular server. Trying to use a mirror
  * for multiple Discourse servers is not permitted; use separate Mirrors.
  */
-export class Mirror {
+export class Mirror implements DiscourseData {
   +_db: Database;
   +_fetcher: Discourse;
 
@@ -168,11 +200,6 @@ export class Mirror {
       }));
   }
 
-  /**
-   * Given a TopicId and a post number, find that numbered post within the topic.
-   *
-   * Returns undefined if no such post exists.
-   */
   findPostInTopic(topicId: TopicId, indexWithinTopic: number): ?PostId {
     return this._db
       .prepare(