blame: unbreak "diff -U 0".

blame: unbreak "diff -U 0".

The commit 604c86d15bb319a1e93ba218fca48ce1c500ae52 changed the
original "diff -u0" to "diff -u -U 0" for portability.

A big mistake without proper testing.

The form "diff -u -U 0" shows the default 3-line contexts,
because -u and -U 0 contradicts with each other; "diff -U 0" (or
its longhand "diff --unified=0") is what we meant.

Signed-off-by: Junio C Hamano <junkio@cox.net>