[MERGE] improve new branch from a smart server

[Andrew Bennetts]

Robert Collins wrote:
> Making a new branch from a smart server was generating a full
> revision-history when it wasn't needed, this is expensive (particularly
> on long lived projects).

bb:tweak

I say "tweak" just because I wouldn't mind a test showing that RemoteBranch
doesn't fetch the full revision history with this method.  IIRC, it shouldn't be
too hard to do along the lines of the existing test_remote tests.

This is clearly a good (and simple) change though.  I won't mind much if you
decide to just merge as-is if you think the effort of writing such a test would
be disproportionate to benefit it would give.

-Andrew.