###### 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).
Style was unidiomatic for modern Nixpkgs, there was no
nativeBuildInputs/buildInputs separation, and some dependencies were
unused.
I've removed doCheck here, because the default Nixpkgs checkPhase
wasn't actually running the tests (need to cd into tests first), and
when I did write a checkPhase to run the tests, they didn't work and I
couldn't make them work.
A note on RCS: There is code in cvsconvert that calls RCS, but I
analyzed it quite carefully, and came to the conclusion that all such
code is unreachable, so I removed the RCS dependency.
Shared permissions are forbidden in the Nix build sandbox which is why
some of the tests fail. Dropping the POSIXPERM test prerequisite, that
is added unconditionally on Linux, should help us avoiding uninteresting
test failures after Git updates. The drawback is that this disables a
few additional tests (as of Git 2.32.0 approx. 71 out of more than 20k
subtests require POSIXPERM).
