diff: fix modified lines stats with --stat and --numstat

Only skip diffstats when both oids are valid and identical. This check
was causing both false-positives (files included in diffstats with no
actual changes (0 lines modified) and false-negatives (showing 0 lines
modified in stats when files had actually changed).

Also replaced same_contents with may_differ to avoid confusion.

Signed-off-by: Thomas Guyot-Sionnest <tguyot@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t3206-range-diff.sh b/t/t3206-range-diff.sh
index e024cff..6eb344b 100755
--- a/t/t3206-range-diff.sh
+++ b/t/t3206-range-diff.sh
@@ -252,17 +252,13 @@
 	git range-diff --no-color --stat topic...changed >actual &&
 	cat >expect <<-EOF &&
 	1:  $(test_oid t1) = 1:  $(test_oid c1) s/5/A/
-	     a => b | 0
-	     1 file changed, 0 insertions(+), 0 deletions(-)
 	2:  $(test_oid t2) = 2:  $(test_oid c2) s/4/A/
-	     a => b | 0
-	     1 file changed, 0 insertions(+), 0 deletions(-)
 	3:  $(test_oid t3) ! 3:  $(test_oid c3) s/11/B/
-	     a => b | 0
-	     1 file changed, 0 insertions(+), 0 deletions(-)
+	     a => b | 2 +-
+	     1 file changed, 1 insertion(+), 1 deletion(-)
 	4:  $(test_oid t4) ! 4:  $(test_oid c4) s/12/B/
-	     a => b | 0
-	     1 file changed, 0 insertions(+), 0 deletions(-)
+	     a => b | 2 +-
+	     1 file changed, 1 insertion(+), 1 deletion(-)
 	EOF
 	test_cmp expect actual
 '