In [#5046], it was discovered that one of the sources of slow queries against the repo_commitrun table was
getmore operations against very large
CommitRunDoc records, which are almost entirely from SVN repos.
SVN commit histories are entirely linear, so the entire history ends up in a single
CommitRunDoc. This is slow and pointless.
Modify the SVN implementation to not create or use
CommitRunDocs, instead leveraging the fact that the histories are inherently linear.