git-svn: --follow-parent now works on sub-directories of larger branches

git-svn: --follow-parent now works on sub-directories of larger branches

This means that tracking the path of:

  /another-larger/trunk/thunk/bump/thud inside a repository

would follow:

  /larger-parent/trunk/thunk/bump/thud

even if the svn log output looks like this:
  --------------------------------------------
  Changed paths:
     A /another-larger (from /larger-parent:5)
  --------------------------------------------

Note: the usage of get_log() in git-svn still makes a
an assumption that shouldn't be made with regard to
revisions existing for a particular path.

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