[Bug 1918112] [NEW] Deal with rred shortcomings around empty patch files

[Julian Andres Klode]

Public bug reported:

[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.

** Affects: apt (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: apt (Ubuntu Focal)
     Importance: Undecided
         Status: Won't Fix

** Affects: apt (Ubuntu Groovy)
     Importance: Undecided
         Status: Won't Fix

** Affects: apt (Ubuntu Hirsute)
     Importance: Undecided
         Status: New

** Also affects: apt (Ubuntu Focal)
   Importance: Undecided
       Status: New

** Also affects: apt (Ubuntu Hirsute)
   Importance: Undecided
       Status: New

** Also affects: apt (Ubuntu Groovy)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1918112

Title:
  Deal with rred shortcomings around empty patch files

Status in apt package in Ubuntu:
  New
Status in apt source package in Focal:
  Won't Fix
Status in apt source package in Groovy:
  Won't Fix
Status in apt source package in Hirsute:
  New

Bug description:
  [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.

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