t4151: make sure argument to 'test -z' is given

t4151: make sure argument to 'test -z' is given

88d50724 (am --skip: revert changes introduced by failed 3way merge,
2015-06-06), unlike all the other patches in the series, forgot to
quote the output from "$(git ls-files -u)" when using it as the
argument to "test -z", leading to a syntax error on platforms whose
test does not interpret "test -z" (no other arguments) as testing if
a string "-z" is the null string (which GNU test and test that is
built into bash and dash seem to do).

Note that $(git ls-files -u | wc -l) is deliberately left unquoted,
as some implementations of "wc -l" includes extra blank characters
in its output and cannot be compared as string, i.e. "test 0 = $(...)".

Signed-off-by: Armin Kunaschik <megabreit@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>