semantic and cosmetic cleanups

* magit-interactive-rebase: fix indentation, shorten long line, use
  `when' instead of `if'
* magit--branch-name-at-point: don't use `let', it isn't necessary and
  doesn't improve readability
* magit--branch-for-remote-repo: use `cl-mapcan' instead of
  (remq nil (mapcar ...))
* magit-remote-part-of-branch, magit-branch-no-remote: shorten long line