flaky test continuing rebase

Seen in unrelated change: https://github.com/twosigma/git-meta/pull/515#pullrequestreview-106602311
```
1) rebase continueSubmodules rebase in a sub, was conflicted:
Error: for repo x
In index
for path s expected Submodule(url=a, sha=qs) but got Submodule(url=a, sha=r)
for open submodule s
missing commit qs
HEAD is r but expected qs
In index
unexpected change to q: q
at Object.exports.assertEqualRepoMaps (lib/util/repo_ast_util.js:644:15)
at Object. (lib/util/repo_ast_test_util.js:352:17)
at next (native)
at onFulfilled (node_modules/co/index.js:65:19)
```