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.c

@@ -3663,7 +3663,7 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 {
 	mmfile_t mf1, mf2;
 	struct diffstat_file *data;
-	int same_contents;
+	int may_differ;
 	int complete_rewrite = 0;
 
 	if (!DIFF_PAIR_UNMERGED(p)) {
@@ -3681,12 +3681,14 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 		return;
 	}
 
-	same_contents = oideq(&one->oid, &two->oid);
+	/* saves some reads if true, not a guarantee of diff outcome */
+	may_differ = !(one->oid_valid && two->oid_valid &&
+			oideq(&one->oid, &two->oid));
 
 	if (diff_filespec_is_binary(o->repo, one) ||
 	    diff_filespec_is_binary(o->repo, two)) {
 		data->is_binary = 1;
-		if (same_contents) {
+		if (!may_differ) {
 			data->added = 0;
 			data->deleted = 0;
 		} else {
@@ -3702,7 +3704,7 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 		data->added = count_lines(two->data, two->size);
 	}
 
-	else if (!same_contents) {
+	else if (may_differ) {
 		/* Crazy xdl interfaces.. */
 		xpparam_t xpp;
 		xdemitconf_t xecfg;
@@ -3727,7 +3729,7 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 				diffstat->files[diffstat->nr - 1];
 			/*
 			 * Omit diffstats of modified files where nothing changed.
-			 * Even if !same_contents, this might be the case due to
+			 * Even if may_differ, this might be the case due to
 			 * ignoring whitespace changes, etc.
 			 *
 			 * But note that we special-case additions, deletions,

t/t3206-range-diff.sh

@@ -252,17 +252,13 @@ test_expect_success 'changed commit with --stat diff option' '
 	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
 '