[PATCH] Eliminate random DAG walks during checkout with ra_serf

Hi there,

Currently, ra_serf creates a significantly higher I/O and CPU load
than ra_svn during checkout. The root cause is that for each path,
it will make two DAG requests, first for path_at_co_rev and then for
path_at_last_changed_rev. Because last_changed revisions have
poor locality, this requires DAG walks for "random" revisions.

Luckily, we do already have all the info available and infrastructure
at hand to bypass the second DAG walk for this specific pattern.
Details are in the .log and .patch files.

If there is no objection to it, I'd like to have this patch in 1.9 and
apply this patch around the 15th.

- Stefan^2.