message: Use temp file for git.checkoutFile
diff: import execa from 'execa';
  import { EOL } from 'os';
  +import { file as tmpFile } from 'tmp-promise';
  import { Context } from '../types';
  @@ -211,11 +212,8 @@ export async function checkout(ref: string) {
  return execGitCommand(`git checkout ${ref}`);
  }
  -export async function checkoutFile(
  - ref: string,
  - fileName: string,
  - targetFileName = `${ref}.${fileName}`
  -) {
  +export async function checkoutFile(ref: string, fileName: string) {
  + const { path: targetFileName } = await tmpFile();
  await execGitCommand(`git show ${ref}:${fileName} > ${targetFileName}`);
  return targetFileName;
  }
  