Move `localGit`/`GitDriver` into Git utils (#150)

Summary:
A few reasons for this:
 1. This _is_ a utility, so it makes sense semantically.
 2. This unifies the utilities API; clients like `loadRepository.test`
    don’t have to keep around both a `git` and a `gitUtils`.
 3. Most importantly, further scripts and tests shouldn’t depend on
    `loadRepository` just for `localGit`. Depending on `gitUtils` makes
    much more sense.

(Note that `makeUtils` is no longer dependency-injectable, but that’s
okay; I considered this and favored YAGNI on this one.)

Test Plan:
Existing unit tests pass.

wchargin-branch: move-localgit

src/plugins/git/gitUtils.js

@@ -1,19 +1,37 @@
 // @flow
 
+import {execFileSync} from "child_process";
 import fs from "fs";
 import mkdirp from "mkdirp";
 import path from "path";
 
-import type {GitDriver} from "./loadRepository";
+export interface Utils {
-
+  exec: GitDriver;
-interface Utils {
   head(): string;
   writeAndStage(filename: string, contents: string): void;
   deterministicCommit(message: string): void;
 }
 
-export function makeUtils(git: GitDriver, repositoryPath: string): Utils {
+export type GitDriver = (args: string[], options?: ExecOptions) => string;
+// `ExecOptions` is the type of the second argument to `execFileSync`.
+// See here for details: https://nodejs.org/api/child_process.html
+type ExecOptions = Object;
+
+export function localGit(repositoryPath: string): GitDriver {
+  return function git(args: string[], options?: ExecOptions): string {
+    return execFileSync(
+      "git",
+      ["-C", repositoryPath, ...args],
+      options
+    ).toString();
+  };
+}
+
+export function makeUtils(repositoryPath: string): Utils {
+  const git = localGit(repositoryPath);
   return {
+    exec: git,
+
     head() {
       return git(["rev-parse", "HEAD"]).trim();
     },

src/plugins/git/loadRepository.js

@@ -9,22 +9,9 @@
  */
 // @flow
 
-import {execFileSync} from "child_process";
+import type {GitDriver} from "./gitUtils";
-
 import type {Repository, Hash, Commit, Tree, TreeEntry} from "./types";
-
+import {localGit} from "./gitUtils";
-export type GitDriver = (args: string[], options?: ExecOptions) => string;
-type ExecOptions = Object; // close enough
-export function localGit(repositoryPath: string): GitDriver {
-  return function git(args: string[], options?: ExecOptions): string {
-    // Throws an Error on shell failure.
-    return execFileSync(
-      "git",
-      ["-C", repositoryPath, ...args],
-      options
-    ).toString();
-  };
-}
 
 /**
  * Load a Git repository from disk into memory. The `rootRef` should be

src/plugins/git/loadRepository.test.js

@@ -2,9 +2,8 @@
 
 import tmp from "tmp";
 
-import type {GitDriver} from "./loadRepository";
 import {makeUtils} from "./gitUtils";
-import {localGit, loadRepository} from "./loadRepository";
+import {loadRepository} from "./loadRepository";
 
 const cleanups: (() => void)[] = [];
 afterAll(() => {
@@ -21,34 +20,33 @@ function mkdtemp() {
 
 function createRepository(): {path: string, commits: string[]} {
   const repositoryPath = mkdtemp();
-  const git = localGit(repositoryPath);
+  const git = makeUtils(repositoryPath);
-  const gitUtils = makeUtils(git, repositoryPath);
 
-  git(["init"]);
+  git.exec(["init"]);
 
-  gitUtils.writeAndStage("README.txt", "Amazing physics going on...\n");
+  git.writeAndStage("README.txt", "Amazing physics going on...\n");
-  gitUtils.deterministicCommit("Initial commit");
+  git.deterministicCommit("Initial commit");
-  const commit1 = gitUtils.head();
+  const commit1 = git.head();
 
-  gitUtils.writeAndStage("src/index.py", "import antigravity\n");
+  git.writeAndStage("src/index.py", "import antigravity\n");
-  gitUtils.writeAndStage(
+  git.writeAndStage(
     "src/quantum_gravity.py",
     'raise NotImplementedError("TODO(physicists)")\n'
   );
-  gitUtils.writeAndStage("TODOS.txt", "1. Resolve quantum gravity\n");
+  git.writeAndStage("TODOS.txt", "1. Resolve quantum gravity\n");
-  gitUtils.deterministicCommit("Discover gravity");
+  git.deterministicCommit("Discover gravity");
-  const commit2 = gitUtils.head();
+  const commit2 = git.head();
 
-  gitUtils.writeAndStage(
+  git.writeAndStage(
     "src/quantum_gravity.py",
     "import random\nif random.random() < 0.5:\n  import antigravity\n"
   );
-  gitUtils.deterministicCommit("Solve quantum gravity");
+  git.deterministicCommit("Solve quantum gravity");
-  const commit3 = gitUtils.head();
+  const commit3 = git.head();
 
-  git(["rm", "TODOS.txt"]);
+  git.exec(["rm", "TODOS.txt"]);
-  gitUtils.deterministicCommit("Clean up TODOS");
+  git.deterministicCommit("Clean up TODOS");
-  const commit4 = gitUtils.head();
+  const commit4 = git.head();
 
   return {
     path: repositoryPath,
@@ -88,16 +86,15 @@ describe("loadRepository", () => {
 
   it("works with submodules", () => {
     const repositoryPath = mkdtemp();
-    const git = localGit(repositoryPath);
+    const git = makeUtils(repositoryPath);
-    const gitUtils = makeUtils(git, repositoryPath);
 
     const subproject = createRepository();
 
-    git(["init"]);
+    git.exec(["init"]);
-    git(["submodule", "--quiet", "add", subproject.path, "physics"]);
+    git.exec(["submodule", "--quiet", "add", subproject.path, "physics"]);
-    gitUtils.deterministicCommit("Initial commit");
+    git.deterministicCommit("Initial commit");
 
-    const head = gitUtils.head();
+    const head = git.head();
 
     const repository = loadRepository(repositoryPath, "HEAD");
     const commit = repository.commits.get(head);