svk improvements on large repository

FYI, the latest svn::mirror commit:

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r309 | clkao | 2005-07-22 15:46:40 +0100 (Fri, 22 Jul 2005) | 10 lines

Refactor find_local_rev to use binary search instead of 
crawling with svn fs history API.

This makes cold cache time on svk info/smerge/switch about
10 times faster on repository with many revisions and projects
with many merge tickets.

This work is sponsored by Fotango LTD.

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It makes on svk test fail, which is harmless after some diagnosis,
and will be fxied soon.

Cheers,
CLK