git-svn.perl: fix a false-positive in the "already exists" test

git-svn.perl: fix a false-positive in the "already exists" test

open_or_add_dir checks to see if the directory already exists or not.
If it already exists and is not a directory, then we fail.  However,
open_or_add_dir did not previously account for the possibility that the
path did exist as a file, but is deleted in the current commit.

In order to prevent this legitimate case from failing, open_or_add_dir
needs to know what files are deleted in the current commit.
Unfortunately that information has to be plumbed through a couple of
layers.

Signed-off-by: Steven Walter <stevenrwalter@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>