Add a "commits" attachment to "differential.diff.search" for retrieving local commit information

Summary:
Ref T13124. See PHI593.

When you arc diff in a Git or Mercurial repository, we upload some information about the local commits in your working copy which the change was generated from.

In the future (for example, with T1508) we may increase the prominence of this feature.

Provide a stable way to read this information back via the API. This roughly mirrors the information we provide about commits in "diffusion.commit.search", although the latter is less fleshed-out today.

Test Plan: Used differential.diff.search to retrieve commit information about Git, Mercurial, and Subversion diffs. (There's no info for Subversion, but it doesn't crash or anything.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13124

Differential Revision: https://secure.phabricator.com/D19386