contrib/git-svn: use refs/remotes/git-svn instead of git-svn-HEAD

contrib/git-svn: use refs/remotes/git-svn instead of git-svn-HEAD

After reading a lengthy discussion on the list, I've come to the
conclusion that creating a 'remotes' directory in refs isn't
such a bad idea.

You can still branch from it by specifying remotes/git-svn (not
needing the leading 'refs/'), and the documentation has been
updated to reflect that.

The 'git-svn' part of the ref can of course be set to whatever
you want by using the GIT_SVN_ID environment variable, as
before.

I'm using refs/remotes/git-svn, and not going with something
like refs/remotes/git-svn/HEAD as it's redundant for Subversion
where there's zero distinction between branches and directories.

Run git-svn rebuild --upgrade to upgrade your repository to use
the new head.  git-svn-HEAD must be manually deleted for safety
reasons.

Side note: if you ever (and I hope you never) want to run
git-update-refs on a 'remotes/' ref, make sure you have the
'refs/' prefix as you don't want to be clobbering your
'remotes/' in $GIT_DIR (where remote URLs are stored).

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