pretty: use starts_with() to check for a prefix

Simplify the code and avoid duplication by using starts_with() instead
of strlen() and strncmp() to check if a line starts with "encoding ".

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

pretty.c

@@ -586,7 +586,7 @@ static char *replace_encoding_header(char *buf, const char *encoding)
 	char *cp = buf;
 
 	/* guess if there is an encoding header before a \n\n */
-	while (strncmp(cp, "encoding ", strlen("encoding "))) {
+	while (!starts_with(cp, "encoding ")) {
 		cp = strchr(cp, '\n');
 		if (!cp || *++cp == '\n')
 			return buf;