Autocomplete on directory path causes hang when directory contains large number of files.

Hi,

If for example, you are using a command like `pandas.read_csv("../dir/subdir/")`. If you are using tab to autocomplete the directory, and `subdir` in this example has a large number of files in it, if you tab again after `subdir/` it can cause the notebook to hang, presumably due to it trying to generate and display a large list of files.

Reproduce the hang in a Jupyter Notebook by using tab completion after a directory path containing many files, such as the `../dir/subdir/` example. Investigate the notebook's path-autocomplete entry point and determine how large result lists are generated and displayed. Done means completion no longer hangs for large directories, with a regression test if the relevant test location can be identified.

Written by the indexing model from the issue text.