git-svn: stop using path names as refnames with --follow-parent

git-svn: stop using path names as refnames with --follow-parent

Using path names as refnames breaks horribly if a user is
tracking one large, toplevel directory, and a lower-level
directory is followed from another project is a parent
of another ref, as it will cause refnames such as:
'refs/remotes/trunk/path/to/stuff', which will conflict
with a refname of 'refs/remotes/trunk'.

Now we just append @$revno to the end of it the current
refname.  And if we have followed back to a grandparent, then
we'll strip any existing '@$parent_revno' strings before
appending our own '@$revno' string to it.

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