Don't diff empty tree on branch creation in paranoid update hook

Don't diff empty tree on branch creation in paranoid update hook

Listing all files in a branch during branch creation is silly;
the user's file-level ACLs probably don't mean anything at this
point.  We now treat the base case of 0{40} as an empty diff,
as this happens only when the user is creating the branch and
there are file level ACLs that diff against the old value of
the branch.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>