Add default branch, description, and metrics (commit count, recent commit) to "diffusion.repository.search"

Summary: Fixes T13430. Provide more information about repositories in "diffusion.repository.search".

Test Plan: Used API console to call method (with new "metrics" attachment), reviewed output. Saw new fields returned.

Maniphest Tasks: T13430

Differential Revision: https://secure.phabricator.com/D20862