Add is_regex_special()

Add is_regex_special(), a character class macro for chars that have a
special meaning in regular expressions.

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

grep.c

@@ -28,16 +28,9 @@ void append_grep_pattern(struct grep_opt *opt, const char *pat,
 	p->next = NULL;
 }
 
-static int isregexspecial(int c)
-{
-	return c == '\0' || is_glob_special(c) ||
-		c == '$' || c == '(' || c == ')' || c == '+' ||
-		c == '.' || c == '^' || c == '{' || c == '|';
-}
-
 static int is_fixed(const char *s)
 {
-	while (!isregexspecial(*s))
+	while (*s && !is_regex_special(*s))
 		s++;
 	return !*s;
 }