grep: factor test for \0 in grep patterns into a function

grep: factor test for \0 in grep patterns into a function

Factor the test for \0 in grep patterns into a function. Since commit
9eceddeec6 ("Use kwset in grep", 2011-08-21) any pattern containing a
\0 is considered fixed as regcomp() can't handle it.

This change makes later changes that make use of either has_null() or
is_fixed() (but not both) smaller.

While I'm at it make the comment conform to the style guide, i.e. add
an opening "/*\n".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>