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>