Commands requiring a work tree must not run in GIT_DIR

This patch helps when you accidentally run something like git-clean
in the git directory instead of the work tree.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-sh-setup.sh

@@ -48,7 +48,8 @@ cd_to_toplevel () {
 }
 
 require_work_tree () {
-	test $(is_bare_repository) = false ||
+	test $(is_bare_repository) = false &&
+	test $(git-rev-parse --is-inside-git-dir) = false ||
 	die "fatal: $0 cannot be used without a working tree."
 }