mailinfo: use strcmp() for string comparison

The array header is defined as:

	static const char *header[MAX_HDR_PARSED] = {
	     "From","Subject","Date",
	};

When looking for the index of a specfic string in that array, simply
use strcmp() instead of memcmp().  This avoids running over the end of
the string (e.g. with memcmp("Subject", "From", 7)) and gets rid of
magic string length constants.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mailinfo.c

@@ -334,7 +334,7 @@ static int check_header(const struct strbuf *line,
 	}
 	if (!prefixcmp(line->buf, "[PATCH]") && isspace(line->buf[7])) {
 		for (i = 0; header[i]; i++) {
-			if (!memcmp("Subject", header[i], 7)) {
+			if (!strcmp("Subject", header[i])) {
 				handle_header(&hdr_data[i], line);
 				ret = 1;
 				goto check_header_out;
@@ -929,13 +929,13 @@ static void handle_info(void)
 		else
 			continue;
 
-		if (!memcmp(header[i], "Subject", 7)) {
+		if (!strcmp(header[i], "Subject")) {
 			if (!keep_subject) {
 				cleanup_subject(hdr);
 				cleanup_space(hdr);
 			}
 			output_header_lines(fout, "Subject", hdr);
-		} else if (!memcmp(header[i], "From", 4)) {
+		} else if (!strcmp(header[i], "From")) {
 			cleanup_space(hdr);
 			handle_from(hdr);
 			fprintf(fout, "Author: %s\n", name.buf);