git-svn: reduce memory usage for large commits

git-svn: reduce memory usage for large commits

apply_textdelta and send_stream can use a separate pool from the
rest of the editor interface, so we'll use a separate SVN::Pool
for them and clear the pool after each file is sent to SVN.

This drastically reduces memory usage per-changeset committed,
and makes large commits (and initial imports) of several
thousand files possible.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>