Merge branch 'nd/clone-detached'

Merge branch 'nd/clone-detached'

* nd/clone-detached:
  clone: fix up delay cloning conditions
  push: do not let configured foreign-vcs permanently clobbered
  clone: print advice on checking out detached HEAD
  clone: allow --branch to take a tag
  clone: refuse to clone if --branch points to bogus ref
  clone: --branch=<branch> always means refs/heads/<branch>
  clone: delay cloning until after remote HEAD checking
  clone: factor out remote ref writing
  clone: factor out HEAD update code
  clone: factor out checkout code
  clone: write detached HEAD in bare repositories
  t5601: add missing && cascade