[RFC] checkout --rebase

Hi,

I'm often work on small topic branches, and find myself doing this quite often:

  # on branch master
  $ git checkout um-build
  $ git rebase master

This is horribly inefficient; the first operation takes a _really_ long time to complete, since master updates itself very often and I have to check out an old worktree. So, I work around this issue by doing:

  # on branch master
  $ git checkout -b um-build-2
  $ git cherry-pick ..um-build
  $ git branch -M um-build

... and I scripted it. Perhaps we should get this functionality in core, as `git checkout --rebase` (or something)?

Thanks.