repo-settings: introduce function to clear struct

repo-settings: introduce function to clear struct

We don't provide a way to clear a `struct repo_settings`, and instead
open-code this in `repo_clear()`. This is mixing up concerns and means
that developers have to touch multiple files whenever they add a new
field to the structure in case the associated resources need to be
released.

Provide a new `repo_settings_clear()` function to improve this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>