Looking at the git documentation, the build dependencies are curl, expat, libiconv, openssl, and zlib -- so in some sense we are adding (at least a subset) of these as dependencies of Sage.
Curl and expat are only used for the http/https protocols, so git will just build without this functionality if those are missing, so they aren't strictly dependencies. We already build zlib, so that isn't a problem. For the other two, I don't have anything more than guesses on the functionality they are needed for.