Speed up log fetches

Issue Summary

Log fetching in the Subversion backend is currently dog slow. That's because 'svn log' doesn't tell us much of the information that we want to provide to the Version Control API:

• It doesn't tell us if a path item is a file or a directory.
• It doesn't tell us the last revision when an item was last changed.
• If an item was deleted or modified and its parent directory has been moved since the last change, it doesn't tell us the previous path of that item.

That makes it necessary to call 'svn info' multiple times, and that makes the log fetching process really slow. We don't really want to miss out on any of the information, but there are a few possibilities left how it could be made faster:

• Given a start state, we could track the item information by ourselves instead of querying 'svn info'. That wouldn't always work, except if the log starts from revision 1 - in that case only "A" entries need to be queried. However, starting from revision 1 is likely the most expensive fetch (the initial import) so the gains are more important there anyways. Also, even if only some tracked items can be reused instead of calling 'svn info', the procedure would be faster still. In order to track items, it would be necessary to find out the sort direction of the input revisions (ascending or descending), sort them for processing, and possibly revert the sorting when processing is finished.
• It would be possible to make svnlib_info() take more detailed parameters: instead of having one fixed $url_revision parameter, there could be a differently looking $repository_urls array that looks like array(array('url' => $url, 'url_revision' => $url_revision), ...), and each of those would be passed to one single 'svn info' invocation. That of course doesn't make sense in combination with the '-r' option (which is used to retrieve previous item states), but we could bundle all 'current item' states in one go. I would guess that makes a third or (at maximum) half of the 'svn info' calls, and should speed up every log fetch considerably.

Those two approaches combined will probably make log fetches reasonably fast... I think so, at least. Other good ideas appreciated, and if someone wants to take a shot at this, you can find the sources in versioncontrol_svn/svnlib/svnlib-deluxe.inc. It's independent of Drupal so you can easily test it with the versioncontrol_svn/svnlib/bin/svnlib-detailed-log.php script. Your chance for instant rockstardom!