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/gitcmd/pipe.go

@@ -8,11 +8,6 @@ import (
 	"os"
 )
 
-type PipeBufferReader interface {
-	Read(p []byte) (n int, err error)
-	Bytes() []byte
-}
-
 type PipeBufferWriter interface {
 	Write(p []byte) (n int, err error)
 	Bytes() []byte