completion: hide dotfiles by default for path completion

completion: hide dotfiles by default for path completion

The previous implementation required callers to explicitly pass a
"hide-dotfiles" flag to __git_complete_index_file to avoid cluttering
completions with hidden files. This led to inconsistent behavior across
commands (e.g., `git add` and `git mv` behaved differently) and forced
callers to maintain repetitive logic.

As suggested by Junio C Hamano, this commit simplifies the logic:
1. __git_complete_index_file now unconditionally hides dotfiles when
   no match pattern is provided.
2. The awk loop in __git_index_files is refactored to check the dotfile
   condition in a single, obvious place after handling path dequoting,
   removing the previous duplication.
3. Callers no longer need to pass "hide-dotfiles".

This provides a cleaner API and ensures a consistent, expected behavior
where dotfiles are hidden unless explicitly requested by typing a dot.

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