log-tree: make name_decoration hash static

In the previous commit, we made add_name_decoration global
so that adders would not have to access the hash directly.
We now make the hash itself static so that callers _have_ to
add through our function, making sure that all additions go
through a single point.  To do this, we have to add one more
accessor function: a way to lookup entries in the hash.

Since the only caller doesn't actually look at the returned
value, but rather only asks whether there is a decoration or
not, we could provide only a boolean "has_name_decoration".
That would allow us to make "struct name_decoration" local
to log-tree, as well.

However, it's unlikely to cause any maintainability harm
making the actual data public, and this interface is more
flexible if we need to look at decorations from other parts
of the code in the future.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>