collection-status par2+file:// crashes (lp:#1635942)

[Original report](https://bugs.launchpad.net/bugs/1635942) created by **David Grajal (dgrabla)**
```
Duplicity version 0.7.10
Python version 2.7.9
OS Distro and version Debian 8.6
Type of target filesystem: Linux

I noticed the collection-status command is not able to detect/list complete
chains if the par2+file:// schema is used. If a cleanup command is run, no
complete chain is detected and the whole backup is flag (and deleted if a
cleanup --force is run). The collection-status command has no trouble if
the exact same backup was done with file:// instead.


On a folder containing a backup made with par2+file:// collection-status
cannot find the chain
```
> duplicity collection-status par2+file://.
Local and Remote metadata are synchronized, no sync needed.
Warning, found incomplete backup sets, probably left from aborted session
Last full backup date: none
Collection Status
-----------------
Connecting with backend: BackendWrapper
Archive dir:
/home/dgrabla/.cache/duplicity/e4858c724edc4a2aab4355784da9ba88

Found 0 secondary backup chains.
No backup chains with active signatures found
Also found 0 backup sets not part of any chain,
and 1 incomplete backup set.
These may be deleted by running duplicity with the "cleanup" command.
```

If the backup was done with transport file:// (without par2) the
collection-status is able to find complete chains:
```
> duplicity collection-status file://.
Synchronizing remote metadata to local cache...
Last full backup date: Sun Oct 23 03:25:54 2016
Collection Status
-----------------
Connecting with backend: BackendWrapper
Archive dir:
/home/dgrabla/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364

Found 0 secondary backup chains.

Found primary backup chain with matching signature chain:
-------------------------
Chain start time: Sun Oct 23 03:25:54 2016
Chain end time: Sun Oct 23 10:26:12 2016
Number of contained backup sets: 7
Total number of contained volumes: 103
 Type of backup set:                            Time:      Num volumes:
                Full         Sun Oct 23 03:25:54 2016                97
         Incremental         Sun Oct 23 05:25:52 2016                 1
         Incremental         Sun Oct 23 06:25:50 2016                 1
         Incremental         Sun Oct 23 07:25:54 2016                 1
         Incremental         Sun Oct 23 08:25:57 2016                 1
         Incremental         Sun Oct 23 09:26:02 2016                 1
         Incremental         Sun Oct 23 10:26:12 2016                 1
-------------------------
No orphaned or incomplete backup sets found.
```

If on the folder with the backup made using transport par2+file:// we try
to do the collection status with file:// (omiting the par2 part) duplicity
crashes

```
> duplicity collection-status file://.
Local and Remote metadata are synchronized, no sync needed.
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1553, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1547, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1398, in main
    do_backup(action)
  File "/usr/bin/duplicity", line 1423, in do_backup
    globals.archive_dir).set_values()
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line
710, in set_values
    self.get_backup_chains(partials + backend_filename_list)
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line
836, in get_backup_chains
    add_to_sets(f)
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line
824, in add_to_sets
    if set.add_filename(filename):
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line
105, in add_filename
    (self.volume_name_dict, filename)
AssertionError: ({4: 'duplicity-
full.20161023T082638Z.vol4.difftar.gpg.par2'}, 'duplicity-
full.20161023T082638Z.vol4.difftar.gpg.vol000+200.par2')
```
```
Original tags: 0.7.10 collection-status

Story points: 6.0