This avoids cp -a overriding the default mode acls that the user might have
set in a git repository.
With GNU cp, this behavior change should not be a breaking change, because
git-anex also uses rsync sometimes in the same situation, and has only ever
preserved timestamps when using rsync.
Systems without GNU cp will no longer use cp -a, but instead just cp.
So, timestamps will no longer be preserved. Preserving timestamps when
copying between repos is not guaranteed anyway.
Closes: #729757
Amoung other things, this makes unlocking a WORM backed file and then
re-adding it without making any changes not add a new object, as the
timestamp is preserved.
This drops the >>! and >>? with the nice low fixity. IfElse does have
undocumented >>=>>! and >>=>>? operators, but I deem that too fishy.
Anyway, using whenM and unlessM is easier; I sometimes mixed the operators
up.
