git-svn: implement auto-discovery of branches/tags

git-svn: implement auto-discovery of branches/tags

This is similar to the way git proper handles refs, except we
use the keys 'branches' and 'tags' to distinguish when we want
to use wildcards.

The left-hand side of the ':' contains the remote path, and must
have one asterisk ('*') in it for the branch name.  The asterisk
may be in any component of the path as long as is it on its own
directory level.

The right-hand side contains the refname and must have the
asterisk as the last path component.

        branches = branches/*:refs/remotes/*
        tags = tags/*:refs/remotes/tags/*

Signed-off-by: Eric Wong <normalperson@yhbt.net>