environment: stop storing "core.warnAmbiguousRefs" globally

environment: stop storing "core.warnAmbiguousRefs" globally

Same as the preceding commits, storing the "core.warnAmbiguousRefs"
value globally is misdesigned as this setting may be set per repository.

Move the logic into the repo-settings subsystem. The usual pattern here
is that users are expected to call `prepare_repo_settings()` before they
access the settings themselves. This seems somewhat fragile though, as
it is easy to miss and leads to somewhat ugly code patterns at the call
sites.

Instead, introduce a new function that encapsulates this logic for us.
This also allows us to change how exactly the lazy initialization works
in the future, e.g. by only partially initializing values as requested
by the caller.

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