Fix another upload-pack deadlock

There was still a deadlock when sending the response failed leaving the writer to stdin blocked.

The deadlock occurs when:

• io.Copy(w, stdout) fails due to the client ('w') going away
• the Git subprocess blocks trying to write to its stdout
• the main goroutine blocks on io.Copy(stdin, buffer)

So the problem this tries to solve is being able to cancel io.Copy(stdin, buffer).