require_work_tree broken with NONGIT_OK

require_work_tree broken with NONGIT_OK

With NONGIT_OK set, require_work_tree function outside a git repository
gives a syntax error.  This is caused by an incorrect use of "test" that
didn't anticipate $(git rev-parse --is-inside-work-tree) may return an
empty string.

Properly quote the argument to "test", and send the standard error stream
to /dev/null to avoid giving duplicate error messages.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>