flaky test in `add.js`

Just saw this fail in the CI build for https://github.com/twosigma/git-meta/pull/513 but I believe it's unrelated to that change and passes elsewhere:

```
1) add stagePaths multiple from sub dirs:
Error: for repo x
In index
missing change to x/y/z: meh
for path x/a/c expected foo but got null
In workdir
unexpected change to x/y/z: meh
at Object.exports.assertEqualRepoMaps (lib/util/repo_ast_util.js:624:15)
at Object. (lib/util/repo_ast_test_util.js:352:17)
at next (native)
at onFulfilled (node_modules/co/index.js:65:19)
```