git-svnimport: support for partial imports

git-svnimport: support for partial imports

This adds support for partial svn imports. Let's assume that SVN
repository layout looks like:

  $trunk/path/to/our/project
  $branches/path/to/our/project
  $tags/path/to/our/project

, and we would like to import only tree under this specific
'path/to/our/project' and not whole tree under $trunk, $branches, etc..
Now we will be be able to do it by using '-P path/to/our/project' option
with git-svnimport.

Signed-off-by: Sasha Khapyorsky <sashak@voltaire.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>