Backup cannot be restored (no signature chains found) (lp:#1086374)

[Original report](https://bugs.launchpad.net/bugs/1086374) created by **Guy Van Sanden (gvs)**
```
I get:

[root@ ]# duplicity list-current-files scp://192.168.11.2/backup/caw-
server1
Local and Remote metadata are synchronized, no sync needed.
Last full backup date: none
Traceback (most recent call last):
File "/usr/bin/duplicity", line 1391, in <module>
with_tempdir(main)
File "/usr/bin/duplicity", line 1384, in with_tempdir
fn()
File "/usr/bin/duplicity", line 1322, in main
list_current(col_stats)
File "/usr/bin/duplicity", line 598, in list_current
sig_chain = col_stats.get_signature_chain_at_time(time)
File "/usr/lib64/python2.6/site-packages/duplicity/collections.py", line
977, in get_signature_chain_at_time
raise CollectionsError("No signature chains found")
CollectionsError: No signature chains found

when trying to list files from my backup.  But the files seem ok outside of
duplicity,
This affects multiple servers backing up to different locations.

OS is CentOS 6.2 x86
Duplicity 0.6.18 from the repos

---
Using archive dir: /root/.cache/duplicity/1dc0bb65e1eb97582ca301487786e1b1
Using backup name: 1dc0bb65e1eb97582ca301487786e1b1
Import of duplicity.backends.ftpsbackend Succeeded
Import of duplicity.backends.u1backend Succeeded
Import of duplicity.backends.ftpbackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.cloudfilesbackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.sshbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.giobackend Succeeded
Import of duplicity.backends.gdocsbackend Succeeded
Main action: list-current
================================================================================
duplicity 0.6.18 (February 29, 2012)
Args: /usr/bin/duplicity -v9 list-current-files --no-encryption
scp://192.168.11.1/backup/caw-server2
Linux caw-server2.cawdekempen.be 2.6.32-220.23.1.el6.x86_64 #1 SMP Mon Jun
18 18:58:52 BST 2012 x86_64 x86_64
/usr/bin/python 2.6.6 (r266:84292, Jun 18 2012, 14:18:47)
[GCC 4.4.6 20110731 (Red Hat 4.4.6-3)]
================================================================================
Local and Remote metadata are synchronized, no sync needed.
0 files exist on backend
0 files exist in cache
Extracting backup chains from list of files: []
Last full backup date: none
Collection Status
-----------------
Connecting with backend: SftpBackend
Archive dir: /root/.cache/duplicity/1dc0bb65e1eb97582ca301487786e1b1

Found 0 secondary backup chains.
No backup chains with active signatures found
No orphaned or incomplete backup sets found.
Using temporary directory /tmp/duplicity-ckTuyx-tempdir
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1391, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1384, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1322, in main
    list_current(col_stats)
  File "/usr/bin/duplicity", line 598, in list_current
    sig_chain = col_stats.get_signature_chain_at_time(time)
  File "/usr/lib64/python2.6/site-packages/duplicity/collections.py", line
977, in get_signature_chain_at_time
    raise CollectionsError("No signature chains found")
CollectionsError: No signature chains found
```

Story points: 50.0