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.go

@@ -161,7 +161,7 @@ func (b *Base) Redirect(location string, status ...int) {
 	}
 	// In case the request is made by "fetch-action" module, make JS redirect to the new location
 	// Otherwise, the JS fetch will follow the redirection and read a "login" page, embed it to the current page, which is not expected.
-	if b.Req.Header.Get("X-Gitea-Fetch-Action") != "" {
+	if httplib.IsGiteaFetchActionRequest(b.Req) {
 		b.JSON(http.StatusOK, map[string]any{"redirect": location})
 		return
 	}