codeintel: Fix integration tests

Created by: efritz

https://github.com/sourcegraph/sourcegraph/commit/6c9ea16c0f48a7bb7835580cb7f5db838102763e introduced a change that causes the integration tests to fail.

We introduced a heuristic to drastically limit the amount of data we need to pull back from gitserver, but that relies on the general truth that uploads for a repository will happen in commit order. This means to find the oldest commit which we care about, we only need to look as far back as the commit for the oldest upload.

This is not true in a test environment. This PR fixes that assumption by uploading files in the order that the commits occur in a topological ordering of the repository's commit graph.