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.
The syntax is ${parameter:-word} (i.e. previously this used
"latestTag" instead of the actual value).
(Fixes a regression from #85278.)
Also: Even though getting the latest tag isn't really security critical
(as long as Git itself is secure against untrusted input), I'd prefer to
switch from the Git to the HTTPS protocol (for authentication of the
server and encryption + uses a standard port).
R. RyanTM