codeSplitters phone page: show a branch's changed lines before Merge #426

The Branches section on #/m/owner/repo (and the repo page) shows a branch's file and op counts, its conflicts and any build-file changes, but not the changed lines themselves. To review an agent's work from a phone before pressing Merge, list each file's ops as a small diff (the review at GET /api/repos/:o/:r/branches/:b already has them). Since a tap is easy to miss on a phone, also consider a confirm step on Merge there.