Fix path duplication in git svn commit-diff

Fix path duplication in git svn commit-diff

Given an SVN repo file:///tmp/svntest/repo, trying to commit changes
to a file proj/trunk/foo.txt in that repo with this command line

  git svn commit-diff -r2 HEAD^ HEAD file:///tmp/svntest/repo/proj/trunk

gave the error message

  Filesystem has no item: File not found: transaction '2-6', path
  '/proj/trunk/proj/trunk/foo.txt'

This fixes the duplication.

Signed-off-by: Karl Hasselström <kha@treskal.com>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>