Do not preserve permissions and acls when copying files from one local git repository to another. Timestamps are still preserved as long as cp --preserve=timestamps is supported.

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
Merge branch 'master' of ssh://git-annex.branchable.com