contrib/git-svn: add -b/--branch switch for branch detection

contrib/git-svn: add -b/--branch switch for branch detection

I've said I don't like branches in Subversion, and I still don't.
This is a bit more flexible, though, as the argument for -b is any
arbitrary git head/tag reference.

This makes some things easier:
 * Importing git history into a brand new SVN branch.
 * Tracking multiple SVN branches via GIT_SVN_ID, even from multiple
   repositories.
 * Adding tags from SVN (still need to use GIT_SVN_ID, though).
 * Even merge tracking is supported, if and only the heads end up with
   100% equivalent tree objects.  This is more stricter but more robust
   and foolproof than parsing commit messages, imho.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>