branch: expose helpers for finding the remote owning a tracking ref

branch: expose helpers for finding the remote owning a tracking ref

The remote-lookup that setup_tracking() does is useful outside
branch.c too; for example, deciding which remote to "git fetch"
from given a remote-tracking ref.

Move 'struct tracking' to branch.h and add two helpers backed by the
existing for_each_remote walk: find_tracking_remote_for_ref() and
advise_ambiguous_fetch_refspec(). setup_tracking() uses both. No
behavior change.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>