only create files_changed when needed in BuildIndexAPIView
only create files_changed when needed in BuildIndexAPIView

Summary: this is an optimization, since getting the list of files changed is a git operation for commit builds.

Test Plan: unit tests + manual testing

Reviewers: paulruan

Reviewed By: paulruan

Subscribers: changesbot, mkedia

Differential Revision: https://tails.corp.dropbox.com/D126187

dropbox/changes,bowlofstew/changes,dropbox/changes,wfxiang08/changes,wfxiang08/changes,wfxiang08/changes,dropbox/changes,bowlofstew/changes,dropbox/changes,bowlofstew/changes,wfxiang08/changes,bowlofstew/changes