Fix failing cache invalidation tests

[kuychaco]

Use repo fixture with multiple commits for tests that check HEAD~

/ cc @daviwil

[smashwilson]

Looking into this a little, I believe these were passing before because the Error was consistent:

{ key: 'getFilePatchForPath {staged, amending} subdir-1/b.txt',
  aSync: 
    { Error
      at new GitError (/Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:32:18)
      at /Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:272:23
      at Generator.next (<anonymous>)
      at step (/Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:20:1)
      at /Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:20:1
      at <anonymous>
    code: 128,
    stdErr: 'fatal: bad revision \'HEAD~\'\n',
    stdOut: '',
    command: 'git -c color.ui=false -c color.status=false -c color.showBranch=false -c color.diff=false -c color.branch=false diff --no-prefix --no-ext-diff --no-renames --diff-filter=u --staged HEAD~ -- subdir-1/b.txt in /private/var/folders/sm/dnqg538d05bb0xmsf4z9vrvc0000gn/T/git-fixture-11817-12625-wa5yr6.qea5' },
 bSync: 
  { Error
      at new GitError (/Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:32:18)
      at /Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:272:23
      at Generator.next (<anonymous>)
      at step (/Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:20:1)
      at /Users/smashwilson/src/atom-github/lib/git-shell-out-strategy.js:20:1
      at <anonymous>
    code: 128,
    stdErr: 'fatal: bad revision \'HEAD~\'\n',
    stdOut: '',
    command: '

The comparisons are done with lodash's toEqual. I'm wondering if we're seeing version drift in lodash.toequal and a change in how strictly it compares Error objects?

(It'd be awfully nice to get apm using a version of npm that respects package-lock.json files...)