Add `github/loadGraph`

This is a replacement for `github/loadGithubData` which returns a
combined Graph rather than a combined RelationalView. This provides a
major benefit, which is that we can use the (robust) Graph merge logic
rather than the (buggy) relational view merge.

Test plan: This function is untested. It basically pipelines a few APIs
together. I think that flow is basically sufficient to validate that it
works, and writing a unit test will be frustrating (mostly will involve
re-integrating the funcitonality via mocks). A future commit makes this
part of the pipeline that generates snapshot tests, so it is de-facto
integration tested.

src/plugins/github/loadGithubData.js

@@ -15,6 +15,7 @@ export type Options = {|
   +cacheDirectory: string,
 |};
 
+// This function is deprecated.
 export async function loadGithubData(options: Options): Promise<void> {
   // We intentionally fetch repositories sequentially rather than in
   // parallel, because GitHub asks that we not make concurrent

src/plugins/github/loadGraph.js

@@ -0,0 +1,48 @@
+// @flow
+
+import {TaskReporter} from "../../util/taskReporter";
+import {createGraph} from "./createGraph";
+import fetchGithubRepo from "./fetchGithubRepo";
+import {RelationalView} from "./relationalView";
+import {type RepoId, repoIdToString} from "../../core/repoId";
+import {Graph} from "../../core/graph";
+
+export type Options = {|
+  +repoIds: $ReadOnlyArray<RepoId>,
+  +token: string,
+  +cacheDirectory: string,
+|};
+
+/**
+ * Loads several GitHub repositories, combining them into a single graph.
+ */
+export async function loadGraph(
+  options: Options,
+  taskReporter: TaskReporter
+): Promise<Graph> {
+  // We intentionally fetch repositories sequentially rather than in
+  // parallel, because GitHub asks that we not make concurrent
+  // requests. From <https://archive.is/LlkQp#88%>:
+  //
+  // > Make requests for a single user or client ID serially. Do not make
+  // > make requests for a single user or client ID concurrently.
+  const repositories = [];
+  for (const repoId of options.repoIds) {
+    const taskId = `github/${repoIdToString(repoId)}`;
+    taskReporter.start(taskId);
+    repositories.push(
+      await fetchGithubRepo(repoId, {
+        token: options.token,
+        cacheDirectory: options.cacheDirectory,
+      })
+    );
+    taskReporter.finish(taskId);
+  }
+  return Graph.merge(
+    repositories.map((r) => {
+      const rv = new RelationalView();
+      rv.addRepository(r);
+      return createGraph(rv);
+    })
+  );
+}