Fix dirmonitor sorting issues (#1599)

* Use `PATHSEP` in path-related functions

* Don't stop on digits when getting the common part in `system.path_compare`

* Avoid sorting multiple times in `dirwatch.get_directory_files`

This also fixes the timeout detection in `recurse_pred`.

src/api/system.c

@@ -1043,7 +1043,7 @@ static int f_load_native_plugin(lua_State *L) {
 #endif
 
 /* Special purpose filepath compare function. Corresponds to the
-   order used in the TreeView view of the project's files. Returns true iff
+   order used in the TreeView view of the project's files. Returns true if
    path1 < path2 in the TreeView order. */
 static int f_path_compare(lua_State *L) {
   size_t len1, len2;
@@ -1057,7 +1057,6 @@ static int f_path_compare(lua_State *L) {
   size_t offset = 0, i, j;
   for (i = 0; i < len1 && i < len2; i++) {
     if (path1[i] != path2[i]) break;
-    if (isdigit(path1[i])) break;
     if (path1[i] == PATHSEP) {
       offset = i + 1;
     }