completion: hide dotfiles for selected path completion

completion: hide dotfiles for selected path completion

The completion helper for index paths uses git ls-files rather than
shell filename completion. As a result, leading-dot paths such as a
tracked .gitignore were offered even when the user had not started the
path with ".".

Hide leading-dot path components for git rm, git mv, and git ls-files
when completing an empty path component. Explicit dot completion is
still preserved, so git rm . can still complete .gitignore.

This matches standard shell filename completion behavior, where dotfiles
are hidden by default unless the user starts their input with a dot.
This also resolves four TODO comments in t/9902-completion.sh which
have been present since 2013 (commit ddf07bddef9a, "completion: add file
completion tests", 2013-04-27), expecting that .gitignore would not be
shown when completing on an empty path component.

Signed-off-by: Zakariyah Ali <zakariyahali100@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>