python3Packages.git-annex-adapter: fix build

`git-annex` version 6.20180719 enhances the output of `git-annex info`
with a description and UUID of the repository which broke the testcase
`test_process_annex_info_batch` which parses the output of the info
command.

The testcase has been fixed accordingly and the patch was filed upstream
here: https://github.com/alpernebbi/git-annex-adapter/pull/6

I rechecked the functionality in a simple python environment with the
following expression:

```
with import ./. {};
python3.withPackages (ps: with ps; [ git-annex-adapter ])
```

Furthermore I tested the main functionality in a Python REPL within the environment.

The Darwin patch fixes tests that rely on `rev`, a binary which is
distributed in the `utillinux` package and not available on Nix with
Darwin.

To make the purpose of the patches for future contributors clear I
documented the purpose for each patch.