Update rdiff-backup and librsync

[fastcat]

Motivation: Update rdiff-backup and underlying librsync library to be compatible with the versions shipped with Debian Bullseye (currently testing), and generally to be up to date with upstream.
Linked issues:

• Takes in the upstream fix to Fix librsync 4GB bug #2433

Checklist

• Build rule all-supported completed successfully
  • for rdiff-backup
  • for duplicity (other user of librsync)
• Package upgrade completed successfully
• New installation of package completed successfully

Note: this is a work in progress that I'm still testing. I'm unsure how to safely complete the "new install" checklist item without putting my NAS that holds my real backups at risk

Similarly I may need help testing that duplicity isn't broken beyond making sure it builds. Debian is building it against librsync 2.0.1, so I expect it should be OK, though it too may need an update to a newer upstream version.

[ymartin59]

Thanks for contribution. Great job.
Some minor improvements are requested. Please also renumber patches so that ordering is properly enforced (at list renumber cross/rdiff-backup/patches/04_librsync2.patch)

[fastcat]

rewriting history to address last review item...

[fastcat]

Now that master is fixed w.r.t. building python 2.7 packages, I've rebased this up to that and smoke tested that it indeed still builds -- I just checked the denverton arch for my NAS, but happy to run a bigger check if that is desired.

(And thanks @ymartin59 for fixing the underlying python issue 🙂)

[fastcat]

I've updated this PR for the new release of rdiff-backup 2.0.0 which has now landed in Debian testing. Unfortunately while the repositories (stored backups) are backwards compatible, the live software is not -- you need 2.0 on both client and server, or neither.

The former patches are all gone now, and it now uses python3 instead of python 2.7.

Edit: This initial attempt at updating to 2.0 isn't working properly. I'm investigating.

[fastcat]

I'm a bit stymied again ... it seems the current python3 package generates a broken virtualenv 'binary' as it's missing the appdirs package and others

[fastcat]

I've addressed the python3 virtualenv issues in #3928, but I'm still having issues with the postinst step here. I can run the steps manually and they work, but they don't seem to be running when installing the package, whether on upgrade or uninstall/reinstall.

[fastcat]

OK, postinst issues fixed, they were my own fault from things I did trying to debug the virtualenv issue

[hgy59]

rdiff-backup is already updated to v2.0.3 and librsync to v2.3.0 with the merge of #3648.

[fastcat]

👍