[RFC] Lazily lookup revision numbers.

[Jelmer Vernooij]

This patch adds a "stores_last_revno()" function to BranchFormat, the
intend of which is to avoid looking up revision numbers if we know a
branch doesn't store them. Instead we'll try to do a lookup using
another branch that does store revno's and can thus retrieve the
revision number much quicker.

At the moment bzr-svn and bzr-git already implement bits of this, by
overriding individual methods in e.g. BranchPullResult and
InterBranch to improve performance. Supporting revno-ignorant branches
in bzrlib a bit better should remove the need for some of these
overrides in bzr-svn/bzr-git/(bzr-hg).

Does this seem like a reasonable approach?

Cheers,

Jelmer
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