storageutil: convert fileid to bytes to avoid cast to %s
test-storage.py manages to trigger this on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D5117
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate