object-name: allow skipping ambiguity checks in `get_oid()` family

object-name: allow skipping ambiguity checks in `get_oid()` family

When reading an object ID via `get_oid_basic()` or any of its related
functions we perform a check whether the object ID is ambiguous, which
can be the case when a reference with the same name exists. While the
check is generally helpful, there are cases where it only adds to the
runtime overhead without providing much of a benefit.

Add a new flag that allows us to disable the check. The flag will be
used in a subsequent commit.

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