original filename is wrongly stored in par2 file

[Marian Sigler]

Since a while (I don't exactly know which version since I skipped some versions due to another bug) when using par2 (specifically, I use par2+pexpect+sftp://), the filename given to par2 includes a path (archive_dir/.../$filename.gpg instead of just $filename.gpg), which causes this error

File is corrupt. Try to repair duplicity-inc.20170418T124107Z.to.20170425T125712Z.manifest.gpg
Failed to repair duplicity-inc.20170418T124107Z.to.20170425T125712Z.manifest.gpg

Demonstrated running `strings` on the par2 files (NAME is the `--name` parameter, obfuscated for privacy; `cache/` is my `--archive-dir`)

earlier (good)
duplicity-inc.20170204T171430Z.to.20170221T140208Z.manifest.gpg.par2: duplicity-inc.20170204T171430Z.to.20170221T140208Z.manifest.gpg
duplicity-inc.20170221T140208Z.to.20170403T131919Z.manifest.gpg.par2: duplicity-inc.20170221T140208Z.to.20170403T131919Z.manifest.gpg

now (bad)
duplicity-inc.20170418T124107Z.to.20170425T125712Z.manifest.gpg.par2: cache/NAME/duplicity_temp.1/duplicity-inc.20170418T124107Z.to.20170425T125712Z.manifest.gpg
duplicity-inc.20170425T125712Z.to.20170508T143651Z.manifest.gpg.par2: cache/NAME/duplicity_temp.1/duplicity-inc.20170425T125712Z.to.20170508T143651Z.manifest.gpg

par2cmdline version 0.7.0
duplicity 0.7.12
Linux 4.10.11-1-ARCH x86_64 GNU/Linux