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.