promisor-remote: add 'local_name' to 'struct promisor_info'

promisor-remote: add 'local_name' to 'struct promisor_info'

In a following commit, we will store promisor remote information under
a remote name different than the one the server advertised.

To prepare for this change, let's add a new 'char *local_name' member
to 'struct promisor_info', and let's update the related functions.

While at it, let's also add a small promisor_info_local_name() helper
that returns `local_name` when set, `name` otherwise, and let's use
this small helper in promisor_store_advertised_fields() and in the
post-loop of filter_promisor_remote() so that lookups against the local
repo configuration use the right name.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>