fix potential infinite loop in S3FS.listdir()
fix potential infinite loop in S3FS.listdir()

The code was accidentally corrupting some internal state used by boto to
manage paging of results.  If the directory contained more than 1000 items,
the first thousand would be listed over and over and over.


git-svn-id: 74b2def6592cf29d88d1a5d33b5c4a2732d8507c@556 67cdc799-7952-0410-af00-57a81ceafa0f

anthonybishopric/pyboxfs,pscottdevos/pyfilesystem,alekibango/pyfilesystem,pscottdevos/pyfilesystem,revolunet/pyfilesystem,simonluijk/pyFilesystem,duedil-ltd/pyfilesystem,alekibango/pyfilesystem,anthonybishopric/pyboxfs