Improve a bit the support for `svn:externals`:    - links for externals with no revision set were using "rev=None" instead of "rev="  - link titles for externals with a revision set were missing a space
Improve a bit the support for `svn:externals`:  
 - links for externals with no revision set were using "rev=None" instead of "rev="
 - link titles for externals with a revision set were missing a space

See also #2340.


git-svn-id: f68c6b3b1dcd5d00a2560c384475aaef3bc99487@5950 af82e41b-90c4-0310-8c96-b1721e28e2e2

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