`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>
