[PATCH] commit --include-externals (v2)

I've rinsed and improved my proposed feature dubbed
  svn commit --include-externals
(Related issues: #1167, #3563)

I hope this will cut a much clearer path through the jungle that is
externals behavior. Now I'm hoping for some reviews!

The idea is to have file and dir externals behave the same way during
commit, and to provide a way to recursively commit all externals (that are
from the same repository and are not revision-pegged).

Who'd have guessed, there are a few corners that would be good to have
others' opinions on.

(To: CMike and Bert because you two were involved in the original
discussion: http://svn.haxx.se/dev/archive-2011-08/0617.shtml )

Thanks!
~Neels