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/git/commit.go

@@ -248,3 +248,14 @@ func GetFullCommitID(ctx context.Context, repo RepositoryFacade, shortID string)
 	}
 	return strings.TrimSpace(commitID), nil
 }
+
+func AddObjectMessageArgument(cmd *gitcmd.Command, typ ObjectType, message string) error {
+	if len(message) > 512*1024 {
+		// It doesn't make sense to store very large messages in git objects,
+		// and it never succeeded in the past due to the command line argument limit (e.g.: 128K on Linux).
+		// If any real world user would complain about the limit, let them explain why, then make the limit configurable.
+		return util.NewInvalidArgumentErrorf("git %s message is too long", typ)
+	}
+	cmd.AddArguments("--file=-").WithStdinBytes([]byte(message))
+	return nil
+}