nix-prefetch-git: fix handling of submodules with spaces

The script would parse the output of `git submodule status` but
didn't handle paths with spaces in them. This would result in the
following error when trying to determine the URL of the submodule:

error: key does not contain a section: .url

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -185,7 +185,7 @@ init_submodules(){
 
         # checkout each submodule
         hash=$(echo "$l" | awk '{print $1}' | tr -d '-')
-        dir=$(echo "$l" | awk '{print $2}')
+        dir=$(echo "$l" | sed -n 's/^ \{0,1\}[^ ]* \(.*\) ([^ ]*)$/\1/p')
         name=$(
             git config -f .gitmodules --get-regexp submodule\..*\.path |
             sed -n "s,^\(.*\)\.path $dir\$,\\1,p")