grep: don't call regexec() for fixed strings

Add the new flag "fixed" to struct grep_pat and set it if the pattern
is doesn't contain any regex control characters in addition to if the
flag -F/--fixed-strings was specified.

This gives a nice speed up on msysgit, where regexec() seems to be
extra slow.  Before (best of five runs):

	$ time git grep grep v1.6.1 >/dev/null

	real    0m0.552s
	user    0m0.000s
	sys     0m0.000s

	$ time git grep -F grep v1.6.1 >/dev/null

	real    0m0.170s
	user    0m0.000s
	sys     0m0.015s

With the patch:

	$ time git grep grep v1.6.1 >/dev/null

	real    0m0.173s
	user    0m0.000s
	sys     0m0.000s

The difference is much smaller on Linux, but still measurable.

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

grep.h

@@ -30,6 +30,7 @@ struct grep_pat {
 	const char *pattern;
 	enum grep_header_field field;
 	regex_t regexp;
+	unsigned fixed:1;
 };
 
 enum grep_expr_node {