ref-filter: make %(upstream:track) prints "[gone]" for invalid upstreams

Borrowing from branch.c's implementation print "[gone]" whenever an
unknown upstream ref is encountered instead of just ignoring it.

This makes sure that when branch.c is ported over to using ref-filter
APIs for printing, this feature is not lost.

Make changes to t/t6300-for-each-ref.sh and
Documentation/git-for-each-ref.txt to reflect this change.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Helped-by : Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -121,7 +121,8 @@ upstream::
 	"[ahead N, behind M]" and `:trackshort` to show the terse
 	version: ">" (ahead), "<" (behind), "<>" (ahead and behind),
 	or "=" (in sync).  Has no effect if the ref does not have
-	tracking information associated with it.
+	tracking information associated with it. `:track` also prints
+	"[gone]" whenever unknown upstream ref is encountered.
 
 push::
 	The name of a local ref which represents the `@{push}` location

ref-filter.c

@@ -1073,8 +1073,10 @@ static void fill_remote_ref_details(struct used_atom *atom, const char *refname,
 		*s = shorten_unambiguous_ref(refname, warn_ambiguous_refs);
 	else if (atom->u.remote_ref == RR_TRACK) {
 		if (stat_tracking_info(branch, &num_ours,
-				       &num_theirs, NULL))
+				       &num_theirs, NULL)) {
+			*s = "[gone]";
 			return;
+		}
 
 		if (!num_ours && !num_theirs)
 			*s = "";

t/t6300-for-each-ref.sh

@@ -382,7 +382,7 @@ test_expect_success 'Check that :track[short] cannot be used with other atoms' '
 
 test_expect_success 'Check that :track[short] works when upstream is invalid' '
 	cat >expected <<-\EOF &&
-
+	[gone]
 
 	EOF
 	test_when_finished "git config branch.master.merge refs/heads/master" &&