diff: refactor object read

Refactor the object reads in diff_populate_filespec() to have the first
object read not be in an if/else branch, because in a future patch, a
retry will be added to that first object read.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>