…ation

To quote the XDG specification:

     There is a single base directory relative to which user-specific
     data files should be written. This directory is defined by the\
     environment variable $XDG_DATA_HOME.

Rather than adding another directory to $HOME, I think that it's better
to follow this standard to avoid a cluttered home-dir.

In contrast to `.service`-units, it's not possible to declare an
`overrides.conf`, however this is done by `generateUnits` for `.nspawn`
units as well. This change breaks the build if you have two derivations
configuring one nspawn unit.

This will happen in a case like this:

``` nix
{ pkgs, ... }: {
  systemd.packages = [
    (pkgs.writeTextDir "etc/systemd/nspawn/container0.nspawn" ''
      [Files]
      Bind=/tmp
    '')
  ];
  systemd.nspawn.container0 = {
    /* ... */
  };
}
```

unifiStable: 5.12.35 -> 5.12.66

tig recently updated it's bash-completion making it depend on __git-complete from git.
Becase __git-complete is not automatically sourced tig bash completion fails.
Also this PR makes tig completion load on-demand.

https://github.com/sagiegurari/cargo-make/releases/tag/0.30.5

humanity-icon-theme: init at 0.6.15

fails to build otherwise.

offlineimap: 7.3.2 -> 7.3.3

nixos/captive-browser: set chromium's data-dir to a XDG-compliant location

nixos/systemd-nspawn: disallow multiple packages with `.nspawn`-units