git: add shell environment for git.git

[zivarah]

This shell.nix file provides an environment that allows building and testing git with make. The tools/libraries included were determined by reviewing git's INSTALL document 1 followed by some trial and error.

Exporting PERL_PATH as an environment variable allows both the build and test processes to work despite perl not being present at /usr/bin/perl on a standard NixOS install.

Tested on both x86_64-linux and aarch64-darwin.