Load .gitignore and exclude regex at time of use

Loading .gitignore and compiling the exclude regex can take more than
15ms. We shouldn't and don't need to pay this cost if we're simply
formatting files given on the command line directly.

I would've loved to lazily import pathspec, but the patch won't be clean
until the file collection and discovery logic is refactored first.

Co-authored-by: Fabio Zadrozny <fabiofz@gmail.com>