Revert change to `require_git(false)` in `WalkBuilder` (#6368)
## Summary
This was changed to fix https://github.com/astral-sh/ruff/issues/5930
(respect `.gitignore` for unzipped source repositories), but led to
undesirable behavior whereby `.gitignore` files in parent directories
are respected regardless of whether you're working in a child git
repository (see: https://github.com/astral-sh/ruff/issues/6335). The
latter is a bigger problem than the former is an important use-case to
support, so pragmatically erring on the side of a revert.
Closes https://github.com/astral-sh/ruff/issues/6335.
