incremental fast-import

[Matt McClure]

I'd like to use bzr fast-import to sync a Bazaar branch from Perforce.
 It works if I do full exports from Perforce, but that doesn't scale
very well.  Instead I'd like to do incremental migrations.  Some
avenues I've explored include:

* fast-import's BadRestart validation
* fast-import's --import-marks option
* the 'from' command specified in the git fast-import documentation

Fast-import's BadRestart validation prevents importing, for example,
revision 3 by itself, to a repository that contains revisions 1 and 2.

Fast-import's --import-marks option raises KeyError if the incremental
import contains no overlapping revisions, e.g., revision 3 to a
repository containing revisions 1 and 2.  On the other hand, if the
incremental import does contain overlapping revisions, e.g. revisions
2 and 3 to a repository containing revisions 1 and 2, then fast-import
creates a duplicate revision in the Bazaar branch for revision 2.