Subversion backend: rewrite the `_history` helper, so that it only uses `svn.fs` primitives. 
Subversion backend: rewrite the `_history` helper, so that it only uses `svn.fs` primitives. 

This is cleaner, as that doesn't involve C <-> Python round-trips via the history_cb and authz_cb. Moreover, `_history` is now a real generator, so it doesn't have to embed the `limit` number of revision logic.



git-svn-id: f68c6b3b1dcd5d00a2560c384475aaef3bc99487@4710 af82e41b-90c4-0310-8c96-b1721e28e2e2

moreati/trac-gitsvn,moreati/trac-gitsvn,dafrito/trac-mirror,dokipen/trac,dokipen/trac,exocad/exotrac,moreati/trac-gitsvn,dafrito/trac-mirror,exocad/exotrac,dokipen/trac,exocad/exotrac,dafrito/trac-mirror,moreati/trac-gitsvn,dafrito/trac-mirror,exocad/exotrac