KnitPackRepository isn't lazy about loading the pack names

[John Arbash Meinel]

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I just noticed this with my other tracing of what was going on for 'bzr
update'.

Specifically, every time you lock_read() a KnitPackRepository it goes
out and re-reads the pack names. The problem with this is that when we
lock a branch (say for getting last_revision_info()) it always locks the
repository. So even though the repository is never accessed, it still
has to read an extra file. That file is typically fairly small
(log(num_revisions) of lines). But it is still an extra probe.

Just something we might want to think about.

John
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