manifest: rewrite pathlen() to not cross entry boundary

Even though the entire manifest data should be terminated by '\0', it seems
not nice to scan '\0' over the entry terminator, '\n'.

mercurial/cext/manifest.c

@@ -42,7 +42,8 b' typedef struct {'
 /* get the length of the path for a line */
 static size_t pathlen(line *l)
 {
-	return strlen(l->start);
+	const char *end = memchr(l->start, '\0', l->len);
+	return (end) ? (size_t)(end - l->start) : l->len;
 }
 
 /* get the node value of a single line */