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do not clean up directories we didn't create clone: factor out dir_exists() helper t5600: modernize style t5600: fix outdated comment about unborn HEAD