notes: allow treeish expressions as notes ref

init_notes() is the main point of entry to the notes API. It ensures
that the input can be used as ref, because it needs a ref to update to
store notes tree after modifying it.

There however are many use cases where notes tree is only read, e.g.
"git log --notes=...".  Any notes-shaped treeish could be used for such
purpose, but it is not allowed due to existing restriction.

Allow treeish expressions to be used in the case the notes tree is going
to be used without write "permissions".  Add a flag to distinguish
whether the notes tree is intended to be used read-only, or will be
updated.

With this change, operations that use notes read-only can be fed any
notes-shaped tree-ish can be used, e.g. git log --notes=notes@{1}.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>