git-svn-synchronizer: clean before re-trying without -s

We first try with the standard trunk/branches/tags layout, but when
that fails, we re-try without that layout. Make sure that the state
is clean before doing so.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
git-svn-synchronizer: do not re-try when we succeeded

In recent versions, git-svn's --stdlayout falls back to the non-default
layout, so we do not have to re-try anymore in that case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>