Install git’s bash completion so that it is loaded on demand

Putting the file in $out/share/bash-completion/completions means that it will be loaded on demand by nixpkgs.bash-completion.

With the old location, the user would either have to explicitly source the file during bash startup, or set BASH_COMPLETION_COMPAT_DIR before sourcing bash_completion.sh, which will eagerly load everything in that directory.