require_work_tree broken with NONGIT_OK

Sourcing git-sh-setup: After having set NONGIT_OK, when calling the function require_work_tree while outside of a git repository shows a syntax error.
This is caused by the call to git rev-parse --is-inside-work-tree printing nothing when it is called outside of a git repository.

Quoting the call removes the syntax error and sending stderr to /dev/null removes duplicate error messages.
---

This is a fixup rollout after discussion on the patch:
- Commit message better identifies the cause of the problem
- the test was not working in v1. fixed this up

 git-sh-setup.sh |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

git-sh-setup.sh +1 -1

index d56426d..5e22440 100755
@@ -128,7 +128,7 @@ cd_to_toplevel () {
 }
  require_work_tree () {
-	test $(git rev-parse --is-inside-work-tree) = true ||
+	test "$(git rev-parse --is-inside-work-tree 2>/dev/null)" = true ||
 	die "fatal: $0 cannot be used without a working tree."
 }
 -- 1.6.6