Fix pool handling in git-svnimport to avoid memory leaks.

Fix pool handling in git-svnimport to avoid memory leaks.

- Create an explicit one-and-only root pool.
- Closely follow examples in SVN::Core man page.
  Before calling a subversion function, create a subpool of our
  root pool and make it the new default pool.
- Create a subpool for looping over svn revisions and clear
  this subpool (i.e. it mark for reuse, don't decallocate it)
  at the start of the loop instead of allocating new memory
  with each iteration.

See http://marc.info/?l=git&m=118554191513822&w=2 for a detailed
explanation of the issue.

Signed-off-by: Stefan Sperling <stsp@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>