This will install the HTML and text documentation into a separate output
so that users can install it without having to rebuild Git.
Previously only `doc/git/git-subtree.html` was installed (which is now
in $doc as well).
The current output sizes are as follows:
```
$ du -sh $(nix-build -A git)
47M /nix/store/wyqgalp61kmavx06rams7z8jz177nd8y-git-2.26.2
$ du -sh $(nix-build -A git.doc)
14M /nix/store/6zi22fl5xc3sg23d9shsviinvwk89wvq-git-2.26.2-doc
```
Fixes#86022 (at least partly since the output has to be installed).
###### Motivation for this change
Update to latest upstream version
###### Things done
* [ ] Tested using sandboxing ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS, or option `sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file) on non-NixOS linux)
* Built on platform(s)
* [x] NixOS
* [ ] macOS
* [ ] other Linux distributions
* [ ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
* [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nix-review --run "nix-review wip"`
* [x] Tested execution of all binary files (usually in `./result/bin/`)
* [ ] Determined the impact on package closure size (by running `nix path-info -S` before and after)
* [ ] Ensured that relevant documentation is up to date
* [x] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).
###### Notify maintainers
cc @worldofpeace @tfc @jtraue
@the-kenny did a good job in the past and is set as maintainer in many package,
however since 2017-2018 he stopped contributing. To create less confusion
in pull requests when people try to request his feedback, I removed him as
maintainer from all packages.
It seems the atom feed now needs authentication. Use the /refs endpoint,
which is used for the switch branch/tag dropdown. It doesn't show all
records, but has some pagination, but works well enough for now.
