builtins.fetchgit: Support a "name" attribute

The "name" attribute defaults to "source", which we should use for all
similar functions (e.g. fetchTarball and in Hydra) to ensure that we
get a consistent store path regardless of how the tree is fetched.

"source" is not necessarily a correct label, but using an empty name
is problematic: you get an ugly store path ending in a dash, and it's
impossible to have a fixed-output derivation that produces that path
because ".drv" is not a valid store name.

Fixes #904.

src/libexpr/primops/fetchgit.hh

@@ -9,6 +9,7 @@ namespace nix {
 class Store;
 
 Path exportGit(ref<Store> store, const std::string & uri,
-    const std::string & ref, const std::string & rev = "");
+    const std::string & ref, const std::string & rev = "",
+    const std::string & name = "");
 
 }