[RFC] Vanilla checkouts in 2.2

[Ian Clatworthy]

The recent thread re branch --bind has highlighted the fact that we
don't have consensus on direction for checkouts. For the record, here's
what I'd like to see us move towards in Bazaar 2.2.

I'd like to see the distinction between heavyweight and lightweight
checkouts go and we simply have checkouts. The --lightweight option
should disappear. In it's place, I'd like to see a --history option that
has one of the following values:

* all - download all history
* 0 - download no history
* revspec - download just back to that revision

So:

  bzr checkout --history all == heavyweight (roughly speaking)
  bzr checkout --history 0   == lightweight
  bzr checkout --history -1  == download just the last revision

I'm not currently fussed on what the default value for the history
option should be. It may even vary depending on the source, e.g. 0 if
local and -10 if remote.

What I do care about more is consistent semantics: a command should
apply to all checkouts or no checkouts at all. So unbind needs to be
either always be legal or never be legal and, if legal, always mean
exactly the same thing. (I guess that would depend on whether we felt
"history" was managed in a bound branch or stacked branch or some new
sort of branch.)

I realise the above is a long way short of solving all the issues around
our user model. OTOH, I think it would be a valuable increase in
functionality (with arguably a decrease in conceptual complexity). And I
think we can make this change without needing to solve other issues
simultaneously.

I'm also not volunteering to take on implementing the above: my time is
pretty limited for numerous reasons. If the idea sounds interesting and
there's some consensus it's worth trying, perhaps someone in the
community could try prototyping it? We could then consider it for 2.2 if
we liked it.

Note: A change like this would need to get agreed to and landed by June
say, leaving enough time for us to update the documentation and iron out
any issues. That's one of the reasons I'm raising this now.

Thoughts?

Ian C.

PS: Thanks to Matthew Fuller for his numerous documents on the Wiki:
http://wiki.bazaar.canonical.com/MatthewFuller. Reading through those
lately has crystallised my thoughts here.