[rfc] [merge] removal of support for reverse changeset application

[Denys Duchier]

Reverse changeset application can just as well be achieved by
reversing the changeset itself (i.e. exchanging the sides) and then
applying it forward.  Aaron concurs.

Removing support for applying changesets in reverse simplifies the
existing code (I believe I also spotted code that was incorrect for
reverse application, but I forgot to write it down, sorry :-/ ).  It
also simplifies the task of directly computing the resulting inventory
from the changeset (so I have a vested interest in this
simplification).

Aaron additionally suggested that I remove support for composing
changesets.

Both these cleanups are available as revision 1492 in the following
branch:

        http://delta.univ-orleans.fr/~duchier/bzr/bzr.cleanup

Cheers,

--Denys