Fixed #10258: handle duplicate file names better.

Instead of just continually appending "_" to duplicate file names, Django's
default storage now appends `_1`, `_2`, `_3`, etc.

Thanks to ianschenck and Thilo.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12552 bcc190cf-cafb-0310-a4f2-bffc1f526a37