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>

modules/web/middleware/cookie.go

@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"strings"
 
+	"gitea.dev/modules/httplib"
 	"gitea.dev/modules/session"
 	"gitea.dev/modules/setting"
 	"gitea.dev/modules/util"
@@ -35,7 +36,7 @@ func DeleteRedirectToCookie(resp http.ResponseWriter) {
 }
 
 func RedirectLinkUserLogin(req *http.Request) string {
-	if req.Header.Get("X-Gitea-Fetch-Action") != "" {
+	if httplib.IsGiteaFetchActionRequest(req) {
 		// when building the redirect link for a fetch request, the current link might be a partial page,
 		// so we only redirect to the login page without redirect_to parameter
 		return setting.AppSubURL + "/user/login"