signature file cropped at 2.1GB (lp:#1882916)

[Original report](https://bugs.launchpad.net/bugs/1882916) created by **torben (crittac)**
```
version: duplicity 0.7.18.2
python: Python 2.7.16
RaspberryPi OS release 2020-05-27
architecture: armv7l GNU/Linux
command: sudo -u www-data PASSPHRASE="" duplicity --encrypt-key GARBELD
/var/www/nextcloud/data/ file://. --exclude "**Music/**" -v9

Issue: the signature files seems to be cut of  at 2.1 GB. Backing up the
same data on 64Bit Ubuntu resulted in a 2.7GB signature file.
The result of this is any other following backups fails with
`File "/usr/lib/python2.7/tarfile.py", line 2352, in next
    raise ReadError("unexpected end of data")
 ReadError: unexpected end of data
`

Logs:
head -n200 /tmp/nc_backup.log

gpg: WARNING: unsafe permissions on homedir '/var/www/.gnupg'
Using archive dir:
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364
Using backup name: db7c0f583d45b95c45d4d76e12d54364
GPG binary is gpg, version 2.2.12
Import of duplicity.backends.acdclibackend Succeeded
Import of duplicity.backends.azurebackend Succeeded
Import of duplicity.backends.b2backend Succeeded
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.cfbackend Succeeded
Import of duplicity.backends.dpbxbackend Failed: No module named dropbox
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.giobackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.hubicbackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.lftpbackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.mediafirebackend Succeeded
Import of duplicity.backends.megabackend Succeeded
Import of duplicity.backends.multibackend Succeeded
Import of duplicity.backends.ncftpbackend Succeeded
Import of duplicity.backends.onedrivebackend Succeeded
Import of duplicity.backends.par2backend Succeeded
Import of duplicity.backends.pydrivebackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.ssh_paramiko_backend Succeeded
Import of duplicity.backends.ssh_pexpect_backend Succeeded
Import of duplicity.backends.swiftbackend Succeeded
Import of duplicity.backends.sxbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
Main action: inc
Acquiring lockfile
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/lockfile
================================================================================
duplicity 0.7.18.2 (October 17, 2018)
Args: /usr/bin/duplicity --encrypt-key E536A8C3 /var/www/nextcloud/data/
file://. --exclude **Music/** -v9
Linux system 4.19.118-v7l+ #1311 SMP Mon Apr 27 14:26:42 BST 2020 armv7l
/usr/bin/python2 2.7.16 (default, Oct 10 2019, 22:02:15)
[GCC 8.3.0]
================================================================================
Using temporary directory /tmp/duplicity-bWiKym-tempdir
Registering (mkstemp) temporary file /tmp/duplicity-bWiKym-tempdir/mkstemp-
sJQtEF-1
Temp has 106878672896 available, backup will use approx 272629760.
Synchronizing remote metadata to local cache...
Deleting local
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/duplicity-full-
signatures.20200608T112308Z.sigtar.gz (not authoritative at backend).
Deleting local
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/duplicity-
full.20200608T112308Z.manifest (not authoritative at backend).
1 file exists on backend
1 file exists in cache
Extracting backup chains from list of files: [u'lost+found']
File lost+found is not part of a known set; creating new set
Ignoring file (rejected by backup set) 'lost+found'
Last full backup date: none
Collection Status
-----------------
Connecting with backend: BackendWrapper
Archive dir: /var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364

Found 0 secondary backup chains.
No backup chains with active signatures found
No orphaned or incomplete backup sets found.
No signatures found, switching to full backup.
Using temporary directory
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/duplicity-
xhQoR2-tempdir
Registering (mktemp) temporary file
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/duplicity-
xhQoR2-tempdir/mktemp-mI0pKK-1
Using temporary directory
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/duplicity-
cj0cFD-tempdir
Registering (mktemp) temporary file
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/duplicity-
cj0cFD-tempdir/mktemp-_9otuh-1
AsyncScheduler: instantiating at concurrency 0
Registering (mktemp) temporary file /tmp/duplicity-bWiKym-
tempdir/mktemp-9REXaB-2
Selecting /var/www/nextcloud/data
Comparing . and None
Getting delta of (. dir) and None
A .
Selection: examining path /var/www/nextcloud/data/.htaccess
Selection:     result: None from function: Command-line exclude glob:
**Music/**
Selection:     + including file
Selecting /var/www/nextcloud/data/.htaccess
Comparing .htaccess and None
Getting delta of (.htaccess reg) and None


tail of log:


File duplicity-full.20200609T163034Z.vol124.difftar.gpg is part of known
set
Found backup chain [Tue Jun  9 17:30:34 2020]-[Tue Jun  9 17:30:34 2020]
--------------[ Backup Statistics ]--------------
StartTime 1591720234.88 (Tue Jun  9 17:30:34 2020)
EndTime 1591757495.26 (Wed Jun 10 03:51:35 2020)
ElapsedTime 37260.39 (10 hours 21 minutes)
SourceFiles 439667
SourceFileSize 201386713287 (188 GB)
NewFiles 439667
NewFileSize 201386713287 (188 GB)
DeletedFiles 0
ChangedFiles 0
ChangedFileSize 0 (0 bytes)
ChangedDeltaSize 0 (0 bytes)
DeltaEntries 439667
RawDeltaSize 201207496903 (187 GB)
TotalDestinationSizeChange 198328583475 (185 GB)
Errors 0
-------------------------------------------------

Releasing lockfile
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/lockfile
Removing still remembered temporary file /tmp/duplicity-bWiKym-
tempdir/mkstemp-sJQtEF-1
Releasing lockfile
/var/www/.cache/duplicity/db7c0f583d45b95c45d4d76e12d54364/lockfile
```

Story points: 6.0