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
osx: Credit @mathiasbynens for OS X settings
bash: Add 'bash_' prefix to custom files

Make the naming consistent. Thanks @alrra