Use default login shell when running git-hooks

[mtsgrd]

We use git2-hooks (gitui) here which in turn runs hooks with bash. This is an issue for most mac users where the default shell is zsh. The hooks will run, but sourcing .bashrc rather than .zshrc.

[Byron]

Note also the related feature-request over at git-ui.

[yerke]

My attempt to fix this issue: https://github.com/extrawurst/gitui/pull/2343