Merge branch 'jk/abort-clone-with-existing-dest'

Merge branch 'jk/abort-clone-with-existing-dest'

"git clone $there $here" is allowed even when here directory exists
as long as it is an empty directory, but the command incorrectly
removed it upon a failure of the operation.

* jk/abort-clone-with-existing-dest:
  clone: do not clean up directories we didn't create
  clone: factor out dir_exists() helper
  t5600: modernize style
  t5600: fix outdated comment about unborn HEAD