fix detection of non-local path, fixes #3108

filenames like ..foobar are valid, so, to detect stuff in upper dirs,
we need to include the path separator and check if it starts with '../'.

(cherry picked from commit 60e924910034b86d3d9c6e9af706e5559cdb4d19)

src/borg/archive.py

@@ -557,7 +557,7 @@ Utilization of max. archive size: {csize_max:.0%}
 
         original_path = original_path or item.path
         dest = self.cwd
-        if item.path.startswith(('/', '..')):
+        if item.path.startswith(('/', '../')):
             raise Exception('Path should be relative and local')
         path = os.path.join(dest, item.path)
         # Attempt to remove existing files, ignore errors on failure