mingw: try to delete target directory before renaming

mingw: try to delete target directory before renaming

When the rename() function tries to move a directory it fails if the
target directory exists. It should check if it can delete the (possibly
empty) target directory and then try again to move the directory.

This partially fixes t9100-git-svn-basic.sh.

Signed-off-by: 마누엘 <nalla@hamal.uberspace.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>