[MERGE][#230567] Faster (local) branch

[Ian Clatworthy]

It was recently reported that the clone time for Bazaar was
very slow (240 secs) compared to Git (11.6 secs) and
Hg (23.8 secs). Branching within a shared repository dropped
the time to 117 secs - better but still lousy.

I've downloaded the repositories used for the benchmark and
tried it on bzr.dev 3448. After packing the repo and taking the
best of 2 times, bzr comes in at 22 secs on my system vs
26 secs for Hg 0.95. Hg was quite a bit faster from cold cache
though so I'm suspecting the Bazaar time quoted (117 secs)
was cold cache. Anyhow, looking at the profile showed we could
do a lot better if we changed how we built the working tree
(in particular).

This patch improves Bazaar's local branch in shared-repo time
from 22 secs to 15.8 secs on a Mozilla tree with 12456 revisions.
It also improves the local branch time on an OpenOffice.org
tree with 305k revisions from 73.4 secs to 41.5 secs.

There's more room for improvement but this seemed a nice point
at which to put the work done so far up for review.

Ian C.

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