chore: remove usages of sort.Sort (#6689)

improve language stats rounding:

- Add tests (I had to omit some edge cases as the current method is
non-determistic in some cases, due to random order of map access).
- Document the algorithm used.
- Lower the amount of calculations that need to be done.
- Because of the aforementioned non-determistic don't use stable sort
and instead regular sort.

better sorting for `RepositoryList`:

- Add testing
- Use `slices.Sortfunc` instead of `sort.Sort`.
- Remove the methods needed for `sort.Sort`.

better git tag sorter:

- Use `slices.SortFunc` instead of `sort.Sort`.
- Remove `tagSorter` and its related methods.
- Added testing.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6689
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

models/repo/language_stats_test.go

@@ -0,0 +1,66 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package repo
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestLanguagePercentages(t *testing.T) {
+	testCases := []struct {
+		input  LanguageStatList
+		output map[string]float32
+	}{
+		{
+			[]*LanguageStat{{Language: "Go", Size: 500}, {Language: "Rust", Size: 501}},
+			map[string]float32{
+				"Go":   50.0,
+				"Rust": 50.0,
+			},
+		},
+		{
+			[]*LanguageStat{{Language: "Go", Size: 10}, {Language: "Rust", Size: 91}},
+			map[string]float32{
+				"Go":   9.9,
+				"Rust": 90.1,
+			},
+		},
+		{
+			[]*LanguageStat{{Language: "Go", Size: 1}, {Language: "Rust", Size: 2}},
+			map[string]float32{
+				"Go":   33.3,
+				"Rust": 66.7,
+			},
+		},
+		{
+			[]*LanguageStat{{Language: "Go", Size: 1}, {Language: "Rust", Size: 2}, {Language: "Shell", Size: 3}, {Language: "C#", Size: 4}, {Language: "Zig", Size: 5}, {Language: "Coq", Size: 6}, {Language: "Haskell", Size: 7}},
+			map[string]float32{
+				"Go":      3.6,
+				"Rust":    7.1,
+				"Shell":   10.7,
+				"C#":      14.3,
+				"Zig":     17.9,
+				"Coq":     21.4,
+				"Haskell": 25,
+			},
+		},
+		{
+			[]*LanguageStat{{Language: "Go", Size: 1000}, {Language: "PHP", Size: 1}, {Language: "Java", Size: 1}},
+			map[string]float32{
+				"Go":    99.8,
+				"other": 0.2,
+			},
+		},
+		{
+			[]*LanguageStat{},
+			map[string]float32{},
+		},
+	}
+
+	for _, testCase := range testCases {
+		assert.Equal(t, testCase.output, testCase.input.getLanguagePercentages())
+	}
+}