svnwrap: improve a docstring & a comment. The docstring for SubversionRepo was technically inaccurate; not only do we require Subversion 1.5, but the reference to a required parameter is inaccurate, as the parameter has a default value. (To be fair, relying on the default value is unlikely to work...) Part of a comment in SubversionRepo.revisions() was redundant, and could be removed. No functionality change.

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