builtins.fetchgit: Support importing a working tree

For example, you can write

  src = fetchgit ./.;

and if ./. refers to an unclean working tree, that tree will be copied
to the Nix store. This removes the need for "cleanSource".

src/libexpr/parser.y

@@ -667,7 +667,7 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
         try {
             if (hasPrefix(elem.second, "git://") || hasSuffix(elem.second, ".git"))
                 // FIXME: support specifying revision/branch
-                res = { true, exportGit(store, elem.second, "master").storePath };
+                res = { true, exportGit(store, elem.second).storePath };
             else
                 res = { true, getDownloader()->downloadCached(store, elem.second, true) };
         } catch (DownloadError & e) {