[MERGE] Support fetching a restricted set of revisions

[Ian Clatworthy]

Jelmer Vernooij wrote:
> The attached bundle adds a parameter to InterRepository.fetch() to
> retrieve a limited set of revisions from the source repository and adds
> an implementation for retrieving a limited set to KnitRepository1. This
> is a step towards shallow branches.

Jelmer,

Exciting to see some progress on this feature!

I appreciate this isn't exposed at the UI yet and the direction you're
going has precedence with the --depth option of git clone, but are we
sure we want to control depth via a numeric limit?

Thinking out loud, is specifying depth by giving a revision spec a
better way? We can easily give a depth of N that way (-N) but it opens
the possibility of specifying the "origin" in more logical ways,
particularly once people start using tags more heavily. For example,
"give me depth back to the version tagged (say) 0.17rc1". It just seems
to me that many people don't want to think about how far they wish to go
back in terms of a count. Well unless that number is 1 and only 1. :-) A
'logical point' in recent project history seems more useful IMO.

I'm asking this because http://bazaar-vcs.org/HistoryHorizon doesn't
seem to make a firm UI decision. And I think the UI will impact the API
down at this level.

Ian C.