Speedup with history-db

[John Arbash Meinel]

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...
>   9.360  fetch up to rev {rgm at gnu.org-20110527071815-pnm36i0e38mb8wte}
>   11.766  history_db post-change-hook took 0.246s (0.002s to get_config, 0.096s to init, 0.148s to import)
>   11.766  Stats:
>   {'num_search_tips': 0,
>    'step mainline': 1,
>    'step mainline added': 4,
>    'step mainline cache missed': 1,
>    'step mainline initial': 1}
>   [ 3688] 2011-05-27 18:06:55.062 INFO: Tree is up to date at revision 104384 of branch bzr+ssh://eliz@bzr.savannah.gnu.org/emacs/trunk
> 
> It looks like something else is at work here.

Actually, it is just confusing names. 'step mainline' is walking the
history of the branch *in the history.db* file. So it is actually
finding that the tip is imported.

I think the issue is that 'bzr up' fires a 'post_branch_tip_changed'
even if the branch doesn't actually change revision. (You can detect
this because old_revision_id == new_revision_id.)

Anyway, bzr-history-db isn't actually adding anything to the cache,
though it seems a bit slow at detecting this. I'll see if I can make it
a bit more of a no-op when nothing has changed.

John
=:->
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