ref-filter: fallback on alphabetical comparison

In ref-filter.c the comparison of refs while sorting is handled by
cmp_ref_sorting() function. When sorting as per numerical values
(e.g. --sort=objectsize) there is no fallback comparison when both
refs hold the same value. This can cause unexpected results (i.e. the
order of listing refs with equal values cannot be pre-determined) as
pointed out by Johannes Sixt ($gmane/280117).

Hence, fallback to alphabetical comparison based on the refname
whenever the other criterion is equal.

A test in t3203 was expecting that branch-two sorts before HEAD, which
happened to be how qsort(3) on Linux sorted the array, but (1) that
outcome was not even guaranteed, and (2) once we start breaking ties
with the refname, "HEAD" should sort before "branch-two" so the
original expectation was inconsistent with the criterion we now use.

Update it to match the new world order, which we can now depend on
being stable.
Helped-by: NJunio C Hamano <gitster@pobox.com>
Reported-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NKarthik Nayak <Karthik.188@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -1483,7 +1483,7 @@ static int cmp_ref_sorting(struct ref_sorting *s, struct ref_array_item *a, stru
 		if (va->ul < vb->ul)
 			cmp = -1;
 		else if (va->ul == vb->ul)
-			cmp = 0;
+			cmp = strcmp(a->refname, b->refname);
 		else
 			cmp = 1;
 	}

t/t3203-branch-output.sh

@@ -145,8 +145,8 @@ EOF
 
 test_expect_success 'git branch `--sort` option' '
 	cat >expect <<-\EOF &&
-	  branch-two
 	* (HEAD detached from fromtag)
+	  branch-two
 	  branch-one
 	  master
 	EOF