completion: address sparse-checkout issues

Correct multiple issues with tab completion of the git sparse-checkout
command. These issues were:

1. git sparse-checkout <TAB> previously resulted in an incomplete list of
subcommands (it was missing reapply and add).
2. Subcommand options were not tab-completable.
3. git sparse-checkout set <TAB> and git sparse-checkout add <TAB> showed
both file names and directory names. While this may be a less surprising
behavior for non-cone mode, cone mode sparse checkouts should complete
only directory names.

Note that while the new strategy of just using git ls-tree to complete on
directory names is simple and a step in the right direction, it does have
some caveats. These are:

1. Likelihood of poor performance in large monorepos (as a result of
recursively completing directory names).
2. Inability to handle paths containing unusual characters.

These caveats will be fixed by subsequent commits in this series.

Signed-off-by: Lessley Dennington <lessleydennington@gmail.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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