pull: pass upload_pack only when it was given

The upload_pack shell variable is initialized to an empty string, so
conditional expansion with ${upload_pack+"$upload_pack"} would not
work very well.  You need a colon there.
Signed-off-by: NJunio 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+"$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