Since we want to replace oid,mode pairs with a single diff_filespec,
we will soon want to be able to use the names 'o', 'a', and 'b' for
the three different file versions. Rename some local variables in
blob_unchanged() that would otherwise conflict.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>