grep: Allow case insensitive search of fixed-strings

"git grep" currently an error when you combine the -F and -i flags.
This isn't in line with how GNU grep handles it.

This patch allows the simultaneous use of those flags.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Brian Collins <bricollins@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.h

@@ -32,6 +32,7 @@ struct grep_pat {
 	enum grep_header_field field;
 	regex_t regexp;
 	unsigned fixed:1;
+	unsigned ignore_case:1;
 	unsigned word_regexp:1;
 };
 
@@ -64,6 +65,7 @@ struct grep_opt {
 	regex_t regexp;
 	int linenum;
 	int invert;
+	int ignore_case;
 	int status_only;
 	int name_only;
 	int unmatch_name_only;