Code review comment for lp:~fullermd/bzr/revspec-dwim

I think Robert's concerns are addressed now in the most simple implementation I could think of.
All we need is a list of revspecs to be tried in a given order (said otherwise,
a revspec is part of the DWIM and when it is, it's tried after some others).

Note that a revspec is needed to implement DWIM and that this revspec is not registered
since the registry requires a prefix.

If w want to add svn:, git: or thread: revisionspec.dwim_revspecs.append() or .insert(0,)
will suffice.

There are alternatives, but I'd like more use cases before addressing them and I'd like
us to *experiment* with the DWIM proposal, as, with all the respect I have for Matthew,
DWIM always need to be tweaked:
- we may want to define _dwim_match_on to replace the dwim_catchable_exceptions,
- we may want to chain revspec classes instead of defining a list (with the idea
  that if you want to add a revspec in the chain you know after or before which revspec
  you want to be tried)

That could be done in another submission.