etc-merge: Refactor and fix dir perms while merging

While merging, existing directory in new_etc was being recursively
deleted which is not correct as any new files might also be deleted.

Instead, we simply create a directory if it doesn't exists, or if it
does exists, we update its metadata accordingly.

Add some test cases for the above.

Signed-off-by: Johan-Liebert1 <[email protected]>

cli: Add internal opt for printing etc-diff

Signed-off-by: Johan-Liebert1 <[email protected]>

Author: Johan-Liebert1