Reduce initial discovery from O(branches * commits) to O(commits)

Summary:
Fixes T4414. Currently, when we discover a new repository, we do something like this:

  foreach (branch) {
    foreach (commit on this branch) {
      do_something();
    }
  }

In cases where there are a lot of branches which mostly just branch `master`, this leads to us doing roughly `O(branches * commits)` work.

We have a `commitCache` to prevent this, but it has two problems:

  - It only fills out of the DB, and we do this whole thing before writing to the DB, which is the entire point.
  - It has a fixed size (2048) and on initial discovery we're usually dealing with way more commits than that.

Instead, also stop doing work if we hit a commit which is known already.

Test Plan:
  - Added `print` on the number of discovered refs and number of unique refs.
  - Ran `bin/repository discover --repair X` on a repo with several branches.
  - Before the patch, got 397 refs and 135 unique refs.
  - After the patch, got 135 refs and 135 unique refs.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4414

Differential Revision: https://secure.phabricator.com/D8374