gitAndTools.stgit: 0.17.1 -> 0.18

Semi-automatic update generated by https://github.com/ryantm/nix-update tools.

This update was made based on information from https://repology.org/metapackage/stgit/versions.

These checks were done:

- built on NixOS
- ran `/nix/store/fwhfsvfs23x3jwc2wgx805x229vxwd24-stgit-0.18/bin/stg -h` got 0 exit code
- ran `/nix/store/fwhfsvfs23x3jwc2wgx805x229vxwd24-stgit-0.18/bin/stg --help` got 0 exit code
- ran `/nix/store/fwhfsvfs23x3jwc2wgx805x229vxwd24-stgit-0.18/bin/stg help` got 0 exit code
- found 0.18 with grep in /nix/store/fwhfsvfs23x3jwc2wgx805x229vxwd24-stgit-0.18
- directory tree listing: https://gist.github.com/b3a9f7bf9ff22a7858ac4f6e4d7d7be0