branch: show "HEAD detached" first under reverse sort

Change the output of the likes of "git branch -l --sort=-objectsize"
to show the "(HEAD detached at <hash>)" message at the start of the
output. Before the compare_detached_head() function added in a
preceding commit we'd emit this output as an emergent effect.

It doesn't make any sense to consider the objectsize, type or other
non-attribute of the "(HEAD detached at <hash>)" message for the
purposes of sorting. Let's always emit it at the top instead. The only
reason it was sorted in the first place is because we're injecting it
into the ref-filter machinery so builtin/branch.c doesn't need to do
its own "am I detached?" detection.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -2357,6 +2357,7 @@ static int cmp_ref_sorting(struct ref_sorting *s, struct ref_array_item *a, stru
 {
 	struct atom_value *va, *vb;
 	int cmp;
+	int cmp_detached_head = 0;
 	cmp_type cmp_type = used_atom[s->atom].type;
 	struct strbuf err = STRBUF_INIT;
 
@@ -2368,6 +2369,7 @@ static int cmp_ref_sorting(struct ref_sorting *s, struct ref_array_item *a, stru
 	if (s->sort_flags & REF_SORTING_DETACHED_HEAD_FIRST &&
 	    ((a->kind | b->kind) & FILTER_REFS_DETACHED_HEAD)) {
 		cmp = compare_detached_head(a, b);
+		cmp_detached_head = 1;
 	} else if (s->sort_flags & REF_SORTING_VERSION) {
 		cmp = versioncmp(va->s, vb->s);
 	} else if (cmp_type == FIELD_STR) {
@@ -2384,7 +2386,8 @@ static int cmp_ref_sorting(struct ref_sorting *s, struct ref_array_item *a, stru
 			cmp = 1;
 	}
 
-	return (s->sort_flags & REF_SORTING_REVERSE) ? -cmp : cmp;
+	return (s->sort_flags & REF_SORTING_REVERSE && !cmp_detached_head)
+		? -cmp : cmp;
 }
 
 static int compare_refs(const void *a_, const void *b_, void *ref_sorting)

t/t3203-branch-output.sh

@@ -221,10 +221,10 @@ test_expect_success 'git branch `--sort=[-]objectsize` option' '
 	test_i18ncmp expect actual &&
 
 	cat >expect <<-\EOF &&
+	* (HEAD detached from fromtag)
 	  branch-one
 	  main
 	  branch-two
-	* (HEAD detached from fromtag)
 	EOF
 	git branch --sort=-objectsize >actual &&
 	test_i18ncmp expect actual
@@ -241,10 +241,10 @@ test_expect_success 'git branch `--sort=[-]type` option' '
 	test_i18ncmp expect actual &&
 
 	cat >expect <<-\EOF &&
+	* (HEAD detached from fromtag)
 	  branch-one
 	  branch-two
 	  main
-	* (HEAD detached from fromtag)
 	EOF
 	git branch --sort=-type >actual &&
 	test_i18ncmp expect actual
@@ -261,10 +261,10 @@ test_expect_success 'git branch `--sort=[-]version:refname` option' '
 	test_i18ncmp expect actual &&
 
 	cat >expect <<-\EOF &&
+	* (HEAD detached from fromtag)
 	  main
 	  branch-two
 	  branch-one
-	* (HEAD detached from fromtag)
 	EOF
 	git branch --sort=-version:refname >actual &&
 	test_i18ncmp expect actual