fetchgitlocal: don't force copying the whole source directory into the store..

pkgs/build-support/fetchgitlocal/default.nix

@@ -1,10 +1,10 @@
 { runCommand, git }: src:
 
 runCommand "local-git-export" {} ''
-  cd ${src}
+  cd ${toString src}
   mkdir -p "$out"
   for file in $(${git}/bin/git ls-files); do
     mkdir -p "$out/$(dirname $file)"
-    cp -d $file "$out/$file"
+    cp -d $file "$out/$file" || true # don't fail when trying to copy a directory
   done
 ''