It's a derogatory term. Not as bad as calling them "bastards" but worse than calling them "swine" :)

Funnily enough, though, "git" is a variant of "get", as in "to beget", and that's a word that theoretically is harmless but when it's used to mean "begotten of", the implication is not nice at all. It's not defined as a bad word but it's acquired that connotation in that particular context through use. Which explains how we got from "get" to "gits" :)