test-lib: do not remove trash_directory if called with --debug

Sometimes you want to keep the trash directory, even if all tests
passed.  For example, when extending tests, it comes it quite handy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -491,7 +491,7 @@ fi
 
 # Test repository
 test="trash directory.$(basename "$0" .sh)"
-remove_trash="$TEST_DIRECTORY/$test"
+test ! -z "$debug" || remove_trash="$TEST_DIRECTORY/$test"
 rm -fr "$test" || {
 	trap - exit
 	echo >&5 "FATAL: Cannot prepare test area"