Deal with rred shortcomings around empty patch files

[Julian Andres Klode]

[Impact]
Merge request https://salsa.debian.org/apt-team/apt/-/merge_requests/159 has a bunch of fixes for empty pdiff files, cleaning up pdiff files, as well as correcting some bugs with index files in the mirror method.

While we don't use pdiffs ourselves, third party repositories might, and people might want to use rred on Ubuntu to create merged pdiff files, and hence benefit from those patches, so we want to backport it to 2.0, such that it is not more buggy than Debian's 1.8 and forces you to run Debian if you want to do those things.

[Test plan]
Each commit comes with a change to the test suite that tests it, and the test suite is fairly exhaustive too.

[Where problems could occur]
Changes are limited to adding headers to Index file downloads (so that mirror works correctly), and pdiff-specific code, so problems could occur with mirror method or pdiff.

[Julian Andres Klode (juliank)]

We're actually not backporting this to stable releases just future stable one :)

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt - 2.2.2

---------------
apt (2.2.2) unstable; urgency=medium

  [ David Kalnischkies ]
  * Deal with rred shortcomings around empty patch files (LP: #1918112)
    - Allow merging with empty pdiff patches
    - Rename pdiff merge patches only after they are all downloaded
    - Start pdiff patching from the last possible starting point
    - Ensure all index files sent custom tags to the methods
  * Harden test for no new acquires after transaction abort (Closes: #984966)

  [ Julian Andres Klode ]
  * Make ADDARG{,C}() macros expand to single statements

 -- Julian Andres Klode <email address hidden> Fri, 12 Mar 2021 09:15:59 +0100