svn clones: fix prefix computation
When using the -s (--stdlayout) option, git-svn tries to be smart and
shorten the base if possible to perhaps grab more history and then glue
the removed part of the base onto the front of each fetch/branches/tags
line.
The computation to determine the actual prefix that git-svn may have added
was incorrect and failed to work properly when there was no prefix added
at all. The new technique first computes the suffix and then removes that
to generate the prefix which always works whether there's a prefix or not.