Intorduce "config edit-ini" sub command to help maintaining INI config file (#35735)

Ref: #32669. Helps addressing
https://gitea.com/gitea/helm-chart/issues/356.

cmd/config_test.go

@@ -0,0 +1,85 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestConfigEdit(t *testing.T) {
+	tmpDir := t.TempDir()
+	configOld := tmpDir + "/app-old.ini"
+	configTemplate := tmpDir + "/app-template.ini"
+	_ = os.WriteFile(configOld, []byte(`
+[sec]
+k1=v1
+k2=v2
+`), os.ModePerm)
+
+	_ = os.WriteFile(configTemplate, []byte(`
+[sec]
+k1=in-template
+
+[sec2]
+k3=v3
+`), os.ModePerm)
+
+	t.Setenv("GITEA__EnV__KeY", "val")
+
+	t.Run("OutputToNewWithEnv", func(t *testing.T) {
+		configNew := tmpDir + "/app-new.ini"
+		err := NewMainApp(AppVersion{}).Run(t.Context(), []string{
+			"./gitea", "--config", configOld,
+			"config", "edit-ini",
+			"--apply-env",
+			"--config-keep-keys", configTemplate,
+			"--out", configNew,
+		})
+		require.NoError(t, err)
+
+		// "k1" old value is kept because its key is in the template
+		// "k2" is removed because it isn't in the template
+		// "k3" isn't in new config because it isn't in the old config
+		// [env] is applied from environment variable
+		data, _ := os.ReadFile(configNew)
+		require.Equal(t, `[sec]
+k1 = v1
+
+[env]
+KeY = val
+`, string(data))
+	})
+
+	t.Run("OutputToExisting(environment-to-ini)", func(t *testing.T) {
+		// the legacy "environment-to-ini" (now a wrapper script) behavior:
+		// if no "--out", then "--in-place" must be used to overwrite the existing "--config" file
+		err := NewMainApp(AppVersion{}).Run(t.Context(), []string{
+			"./gitea", "config", "edit-ini",
+			"--apply-env",
+			"--config", configOld,
+		})
+		require.ErrorContains(t, err, "either --in-place or --out must be specified")
+
+		// simulate the "environment-to-ini" behavior with "--in-place"
+		err = NewMainApp(AppVersion{}).Run(t.Context(), []string{
+			"./gitea", "config", "edit-ini",
+			"--in-place",
+			"--apply-env",
+			"--config", configOld,
+		})
+		require.NoError(t, err)
+
+		data, _ := os.ReadFile(configOld)
+		require.Equal(t, `[sec]
+k1 = v1
+k2 = v2
+
+[env]
+KeY = val
+`, string(data))
+	})
+}