"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t5600: fix outdated comment about unborn HEAD