I don’t know Dreamweaver at all, but it certainly has the ability to know when an underlying file has changed, yes? Suppose you open an html file in Dreamweaver. Suppose you open it also in vim or TextWrangler; were you to save it in vim or TextWrangler, would it throw an alert to reload the file in Dreamweaver?
If so, that’s all changing branches will do.
Unfortunately, Dreamweaver doesn’t have direct git integration the way, say, Eclipse or IntelliJ do.