for-each-ref: always check stat_tracking_info()'s return value

The code handling %(upstream:track) and %(upstream:trackshort)
assumed that it always had a valid branch that had been sanitized
earlier in populate_value(), and thus did not check the return value
of the call to stat_tracking_info().

While there is indeed some sanitization code that basically
corresponds to stat_tracking_info() returning 0 (no base branch
set), the function can also return -1 when the base branch did exist
but has since then been deleted.

In this case, num_ours and num_theirs had undefined values and a
call to `git for-each-ref --format="%(upstream:track)"` could print
spurious values such as

  [behind -111794512]
  [ahead 38881640, behind 5103867]

even for repositories with one single commit.

Verify stat_tracking_info()'s return value and do not print anything
if it returns -1. This behavior also matches the documentation ("has
no effect if the ref does not have tracking information associated
with it").

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/for-each-ref.c

@@ -728,7 +728,10 @@ static void populate_value(struct refinfo *ref)
 				 starts_with(name, "upstream")) {
 				char buf[40];
 
-				stat_tracking_info(branch, &num_ours, &num_theirs);
+				if (stat_tracking_info(branch, &num_ours,
+						       &num_theirs) != 1)
+					continue;
+
 				if (!num_ours && !num_theirs)
 					v->s = "";
 				else if (!num_ours) {
@@ -746,7 +749,11 @@ static void populate_value(struct refinfo *ref)
 			} else if (!strcmp(formatp, "trackshort") &&
 				   starts_with(name, "upstream")) {
 				assert(branch);
-				stat_tracking_info(branch, &num_ours, &num_theirs);
+
+				if (stat_tracking_info(branch, &num_ours,
+							&num_theirs) != 1)
+					continue;
+
 				if (!num_ours && !num_theirs)
 					v->s = "=";
 				else if (!num_ours)

t/t6300-for-each-ref.sh

@@ -334,6 +334,19 @@ test_expect_success 'Check that :track[short] cannot be used with other atoms' '
 	test_must_fail git for-each-ref --format="%(refname:trackshort)" 2>/dev/null
 '
 
+test_expect_success 'Check that :track[short] works when upstream is invalid' '
+	cat >expected <<-\EOF &&
+
+
+	EOF
+	test_when_finished "git config branch.master.merge refs/heads/master" &&
+	git config branch.master.merge refs/heads/does-not-exist &&
+	git for-each-ref \
+		--format="%(upstream:track)$LF%(upstream:trackshort)" \
+		refs/heads >actual &&
+	test_cmp expected actual
+'
+
 cat >expected <<EOF
 $(git rev-parse --short HEAD)
 EOF