fix: pass merge commit messages to git via stdin (#39269)

`git commit --message=` passes the merge message as a single argument,
which Linux caps at 128 KiB and Windows at 32 KiB for the whole command
line. Long messages failed with `argument list too long` and the merge
box toast showed the raw HTML 500 page.

Pass the message via `--file=-` on stdin instead, and answer
fetch-action requests with JSON on server errors so the toast shows the
error text. Limits merge commit messages to 512KB which could be
extended or made configurable later.

Fixes: https://github.com/go-gitea/gitea/issues/39261
Fixes: https://github.com/go-gitea/gitea/issues/30276
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/context/base_test.go

@@ -6,6 +6,7 @@ package context
 import (
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"testing"
 
 	"gitea.dev/modules/setting"
@@ -13,8 +14,12 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-func TestRedirect(t *testing.T) {
+func TestMain(m *testing.M) {
 	setting.IsInTesting = true
+	os.Exit(m.Run())
+}
+
+func TestRedirect(t *testing.T) {
 	req, _ := http.NewRequest(http.MethodGet, "/", nil)
 
 	cases := []struct {