git-diff -B output fix.

Geert noticed that complete rewrite diff missed the usual a/ and b/
leading paths.  Pickaxe says it never worked, ever.

Embarrassing.

Signed-off-by: Junio C Hamano <junkio@cox.net>
(cherry picked from bc1a5807575b2f34538d4158834da6524a4fc1f7 commit)

diff.c

diff --git a/diff.c b/diff.c
index b3b1781..5dbc913 100644
--- a/diff.c
+++ b/diff.c
@@ -333,7 +333,7 @@
 	diff_populate_filespec(two, 0);
 	lc_a = count_lines(one->data, one->size);
 	lc_b = count_lines(two->data, two->size);
-	printf("--- %s\n+++ %s\n@@ -", name_a, name_b);
+	printf("--- a/%s\n+++ b/%s\n@@ -", name_a, name_b);
 	print_line_count(lc_a);
 	printf(" +");
 	print_line_count(lc_b);