mc: 4.8.19 -> 4.8.20

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20/bin/mc -h` got 0 exit code
- ran `/nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20/bin/mc --help` got 0 exit code
- ran `/nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20/bin/mc -V` and found version 4.8.20
- ran `/nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20/bin/mc --version` and found version 4.8.20
- ran `/nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20/bin/mc -h` and found version 4.8.20
- ran `/nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20/bin/mc --help` and found version 4.8.20
- found 4.8.20 with grep in /nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20
- found 4.8.20 in filename of file in /nix/store/1qjgxba6vrvnq70gjwfmhhj0blxlr5fy-mc-4.8.20

cc "@sander"