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>

grep.c

@@ -334,9 +334,15 @@ static int fixmatch(const char *pattern, char *line, char *eol,
 {
 	char *hit;
 
-	if (ignore_case)
-		hit = strcasestr(line, pattern);
-	else
+	if (ignore_case) {
+		char *s = line;
+		do {
+			hit = strcasestr(s, pattern);
+			if (hit)
+				break;
+			s += strlen(s) + 1;
+		} while (s < eol);
+	} else
 		hit = memmem(line, eol - line, pattern, strlen(pattern));
 
 	if (!hit) {

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