t7518: fix flaky grep invocation

t7518.1 added in commit 862e80a413 ("ident: handle NULL email when complaining of empty name", 2017-02-23), was trying to make sure that the test with an empty ident did not segfault and did not result in glibc quiety translating a NULL pointer into a name of "(null)". It did the latter by ensuring that a grep for "null" didn't appear in the output, but on one automatic CI run I observed the following output: fatal: empty ident name (for <runner@fv-az128-670.gcliasfzo2nullsdbrimjtbyhg.cx.internal.cloudapp.net>) not allowed Note that 'null' appears as a substring of the domain name, found within 'gcliasfzo2nullsdbrimjtbyhg'. Tighten the test by searching for "(null)" rather than "null". Signed-off-by: Elijah Newren <newren@gmail.com> Reviewed-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>