Support Git repos in the Nix path

E.g.

  $ nix-build -I nixpkgs=git://github.com/NixOS/nixpkgs '<nixpkgs>' -A hello

This is not extremely useful yet because you can't specify a
branch/revision.

src/libutil/util.cc

@@ -1082,6 +1082,12 @@ bool statusOk(int status)
 }
 
 
+bool hasPrefix(const string & s, const string & suffix)
+{
+    return s.compare(0, suffix.size(), suffix) == 0;
+}
+
+
 bool hasSuffix(const string & s, const string & suffix)
 {
     return s.size() >= suffix.size() && string(s, s.size() - suffix.size()) == suffix;