plugin to reweave history, inserting ghosts

[Martin Pool]

One goal we want to support in bazaar-ng is "skeleton" or "ghost"
revisions (or some other ghoulish name).  This is where a revision
knows some other revision has been merged, but it doesn't know
anything else about it.  This is supported by tla and baz 1.x, but not
by many other systems.  (It's hard or impossible to do in a system
that identifies versions by hashes.)

The immediate reason for supporting them is that many arch archives
have them and we have promised a lossless upgrade from bazaar 1.x to
bazaar-ng.  In any case they can be useful in managing e.g. imports
from other systems.

One thing we need to support this is to be able to combine the view of
history seen by two branches.  A plugin to do this is here:

  http://people.ubuntu.com/~mbp/bzrplugin.mbp.reweave-branches/

This is like aaron's fetch-missing command, but works more deeply,
going back to edit revisions that already exist in the destination
branch.

This is only a plugin because I hope that in the future we can detect
situations where it should be done and do the equivalent operations
automatically.  But here it is for anyone who cares to try it.

--
Martin