Make dirwatch sorting compatible with what file_bisect expects (#1300)

The result of `a.filename < b.filename` is sometimes different from 
`system.path_compare(a.filename, a.type, b.filename, b.type)` which 
causes issues to `file_bisect`, as it expects the sorting to be done 
with `system.path_compare`.

data/core/dirwatch.lua

@@ -166,7 +166,7 @@ end
 
 
 local function compare_file(a, b)
-  return a.filename < b.filename
+  return system.path_compare(a.filename, a.type, b.filename, b.type)
 end