bash: Check if in `.git` dir when building prompt
bash: Check if in `.git` dir when building prompt

Some of the `git` commands will not work when run inside the `.git` dir.
Therefore, need to test if we're in a `.git` directory and then return
early if we are.

Fix gh-8

senorflor/linux-dotfiles,marc0uk/dotfiles,bronxct1/dotfiles,lgaechter/dotfiles,jmyser/dotfiles,seiryu9/dotfiles,micahwood/linux-dotfiles,codfish/dotfiles,pmacom/dotfiles,SiTaggart/dotfiles,micahwood/linux-dotfiles,seiryu9/dotfiles,necolas/dotfiles,SBoudrias/dotfiles,micahwood/linux-dotfiles,pertoyra/dotfiles,Stubby/dotfiles,sdillingham/dotfiles,codfish/dotfiles,saclark/dotfiles,senorflor/dotfiles,bronxct1/dotfiles,micahwood/linux-dotfiles,intiocean/dotfiles,Stubby/dotfiles,Gastove/dotfiles,micahwood/linux-dotfiles