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