contrib/git-svn: polishing up

Mostly small things: documentation and code cleanups.

One bugfix of some significance:
    
    Edited commit messages are parsed back to remotes/git-svn
    correctly.  The sequential commit optimization broke this :x

Significant new features:
    
    --branch/-b is interesting and fun for those of you who miss 'real'
    branch support in SVN.

    --authors-file/-A support, because git-svnimport has it :)

None of the core commit/fetch tree-writing code was touched.

 git-svn.perl |  210 ++++++++++++++++++++++++++++++++++++++++-------------------
 git-svn.txt  |   34 ++++++++-
 2 files changed, 176 insertions(+), 68 deletions(-)