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

grep.c

@@ -28,9 +28,31 @@ void append_grep_pattern(struct grep_opt *opt, const char *pat,
 	p->next = NULL;
 }
 
+static int isregexspecial(int c)
+{
+	return isspecial(c) || c == '$' || c == '(' || c == ')' || c == '+' ||
+			       c == '.' || c == '^' || c == '{' || c == '|';
+}
+
+static int is_fixed(const char *s)
+{
+	while (!isregexspecial(*s))
+		s++;
+	return !*s;
+}
+
 static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 {
-	int err = regcomp(&p->regexp, p->pattern, opt->regflags);
+	int err;
+
+	if (opt->fixed || is_fixed(p->pattern))
+		p->fixed = 1;
+	if (opt->regflags & REG_ICASE)
+		p->fixed = 0;
+	if (p->fixed)
+		return;
+
+	err = regcomp(&p->regexp, p->pattern, opt->regflags);
 	if (err) {
 		char errbuf[1024];
 		char where[1024];
@@ -159,8 +181,7 @@ void compile_grep_patterns(struct grep_opt *opt)
 		case GREP_PATTERN: /* atom */
 		case GREP_PATTERN_HEAD:
 		case GREP_PATTERN_BODY:
-			if (!opt->fixed)
-				compile_regexp(p, opt);
+			compile_regexp(p, opt);
 			break;
 		default:
 			opt->extended = 1;
@@ -314,7 +335,7 @@ static int match_one_pattern(struct grep_opt *opt, struct grep_pat *p, char *bol
 	}
 
  again:
-	if (!opt->fixed) {
+	if (!p->fixed) {
 		regex_t *exp = &p->regexp;
 		hit = !regexec(exp, bol, ARRAY_SIZE(pmatch),
 			       pmatch, 0);

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 {