There's a Google Mail feature you have to use. Seriously. You must. Because copying an entire chain of messages after your reply doesn't make any sense when people can scroll down to see all the messages, chained one after the other. What makes sense is to only provide the snippet that you are actually replying to. And that's why you need to do this:
1. Select the text you are replying to in Gmail.
2. Hit the reply button.
3. Boom! Only the selected text will be quoted. Reply at will.
I know. This is common in all mail programs, but most people don't know it exists in Gmail too. I discovered it by chance a long time ago, assuming it would work, but today I discovered that most people don't know about it.