grep: continue case insensitive fixed string search after NUL chars

Functions for C strings, like strcasestr(), can't see beyond NUL
characters.  Check if there is such an obstacle on the line and try
again behind it.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7008-grep-binary.sh

@@ -55,4 +55,8 @@ test_expect_success 'git grep -F ile a' '
 	git grep -F ile a
 '
 
+test_expect_success 'git grep -Fi iLE a' '
+	git grep -Fi iLE a
+'
+
 test_done