compare svn loader performances with git-svn

We expect svn loading to be slower than git loading in general, but it would be useful to have a comparison baseline to make sure we are not doing worse than the state-of-the-art in svn loading. Comparing the performances of our svn loader with those of git-svn on a medium/large repository is a good way to test that.

(hat tip to @rdicosmo for the suggestion)

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Migrated from T386 (view on Phabricator)