R1:843bfb4fd8ac

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…