pull.sh: quote $upload_pack when passing it to git-fetch

The previous code broke for example

  git pull --upload-pack 'echo --foo'

Reported-by: Joey Hess <id@joeyh.name>
Fix-suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -295,7 +295,7 @@ test true = "$rebase" && {
 }
 orig_head=$(git rev-parse -q --verify HEAD)
 git fetch $verbosity $progress $dry_run $recurse_submodules $all $append \
-$upload_pack $force $tags $prune $keep $depth $unshallow $update_shallow \
+${upload_pack+"$upload_pack"} $force $tags $prune $keep $depth $unshallow $update_shallow \
 $refmap --update-head-ok "$@" || exit 1
 test -z "$dry_run" || exit 0