Extract code to create the example repository (#152)

Test Plan:
Note that the snapshot change is simply a move: no SHAs were changed.

wchargin-branch: extract-example-repository-code

src/plugins/git/__snapshots__/loadRepository.test.js.snap

@@ -133,12 +133,3 @@ Object {
   },
 }
 `;
-
-exports[`we create a deterministic repository 1`] = `
-Array [
-  "301749e9af8cd6e9aee3a49a64029b98a4695e34",
-  "cbb26b570d1eed3c681b8f03ff31231c1bffd6d6",
-  "4be43f1cda04e51e42fec0cfe8e1e2dff116e839",
-  "677b340674bde17fdaac3b5f5eef929139ef2a52",
-]
-`;

src/plugins/git/demoData/__snapshots__/exampleRepo.test.js.snap

@@ -0,0 +1,10 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`createExampleRepo is deterministic 1`] = `
+Array [
+  "301749e9af8cd6e9aee3a49a64029b98a4695e34",
+  "cbb26b570d1eed3c681b8f03ff31231c1bffd6d6",
+  "4be43f1cda04e51e42fec0cfe8e1e2dff116e839",
+  "677b340674bde17fdaac3b5f5eef929139ef2a52",
+]
+`;

src/plugins/git/demoData/exampleRepo.js

@@ -0,0 +1,46 @@
+// @flow
+
+import mkdirp from "mkdirp";
+
+import {makeUtils} from "../gitUtils";
+import type {Hash} from "../types";
+
+type RepositoryInfo = {|
+  +path: string,
+  +commits: $ReadOnlyArray<Hash>, // in oldest-to-newest order
+|};
+
+export function createExampleRepo(intoDirectory: string): RepositoryInfo {
+  const repositoryPath = intoDirectory;
+  mkdirp(repositoryPath);
+  const git = makeUtils(repositoryPath);
+  const commits = [];
+
+  git.exec(["init"]);
+
+  git.writeAndStage("README.txt", "Amazing physics going on...\n");
+  git.deterministicCommit("Initial commit");
+  commits.push(git.head());
+
+  git.writeAndStage("src/index.py", "import antigravity\n");
+  git.writeAndStage(
+    "src/quantum_gravity.py",
+    'raise NotImplementedError("TODO(physicists)")\n'
+  );
+  git.writeAndStage("TODOS.txt", "1. Resolve quantum gravity\n");
+  git.deterministicCommit("Discover gravity");
+  commits.push(git.head());
+
+  git.writeAndStage(
+    "src/quantum_gravity.py",
+    "import random\nif random.random() < 0.5:\n  import antigravity\n"
+  );
+  git.deterministicCommit("Solve quantum gravity");
+  commits.push(git.head());
+
+  git.exec(["rm", "TODOS.txt"]);
+  git.deterministicCommit("Clean up TODOS");
+  commits.push(git.head());
+
+  return {path: repositoryPath, commits};
+}

src/plugins/git/demoData/exampleRepo.test.js

@@ -0,0 +1,24 @@
+// @flow
+
+import tmp from "tmp";
+
+import {createExampleRepo} from "./exampleRepo";
+
+const cleanups: (() => void)[] = [];
+afterAll(() => {
+  cleanups.forEach((f) => {
+    f();
+  });
+});
+
+function mkdtemp() {
+  const result = tmp.dirSync({unsafeCleanup: true});
+  cleanups.push(() => result.removeCallback());
+  return result.name;
+}
+
+describe("createExampleRepo", () => {
+  it("is deterministic", () => {
+    expect(createExampleRepo(mkdtemp()).commits).toMatchSnapshot();
+  });
+});

src/plugins/git/loadRepository.test.js

@@ -2,6 +2,7 @@
 
 import tmp from "tmp";
 
+import {createExampleRepo} from "./demoData/exampleRepo";
 import {makeUtils} from "./gitUtils";
 import {loadRepository} from "./loadRepository";
 
@@ -18,54 +19,14 @@ function mkdtemp() {
   return result.name;
 }
 
-function createRepository(): {path: string, commits: string[]} {
-  const repositoryPath = mkdtemp();
-  const git = makeUtils(repositoryPath);
-
-  git.exec(["init"]);
-
-  git.writeAndStage("README.txt", "Amazing physics going on...\n");
-  git.deterministicCommit("Initial commit");
-  const commit1 = git.head();
-
-  git.writeAndStage("src/index.py", "import antigravity\n");
-  git.writeAndStage(
-    "src/quantum_gravity.py",
-    'raise NotImplementedError("TODO(physicists)")\n'
-  );
-  git.writeAndStage("TODOS.txt", "1. Resolve quantum gravity\n");
-  git.deterministicCommit("Discover gravity");
-  const commit2 = git.head();
-
-  git.writeAndStage(
-    "src/quantum_gravity.py",
-    "import random\nif random.random() < 0.5:\n  import antigravity\n"
-  );
-  git.deterministicCommit("Solve quantum gravity");
-  const commit3 = git.head();
-
-  git.exec(["rm", "TODOS.txt"]);
-  git.deterministicCommit("Clean up TODOS");
-  const commit4 = git.head();
-
-  return {
-    path: repositoryPath,
-    commits: [commit1, commit2, commit3, commit4],
-  };
-}
-
-test("we create a deterministic repository", () => {
-  expect(createRepository().commits).toMatchSnapshot();
-});
-
 describe("loadRepository", () => {
   it("loads from HEAD", () => {
-    const repository = createRepository();
+    const repository = createExampleRepo(mkdtemp());
     expect(loadRepository(repository.path, "HEAD")).toMatchSnapshot();
   });
 
   it("processes an old commit", () => {
-    const repository = createRepository();
+    const repository = createExampleRepo(mkdtemp());
     const whole = loadRepository(repository.path, "HEAD");
     const part = loadRepository(repository.path, repository.commits[1]);
 
@@ -88,7 +49,7 @@ describe("loadRepository", () => {
     const repositoryPath = mkdtemp();
     const git = makeUtils(repositoryPath);
 
-    const subproject = createRepository();
+    const subproject = createExampleRepo(mkdtemp());
 
     git.exec(["init"]);
     git.exec(["submodule", "--quiet", "add", subproject.path, "physics"]);