Rely on personal files over shared files.

With local dotfiles taking precedence in `.rcrc`, you can now override
bin stubs and zsh functions easily.

Changes:

* tat starts a new session with the current directory name if none
  exists
* Don't use rspec and rake with thoughtbot's zeus-related check

Also:
Use command git for git aliases

When using `hub`, running `command git` is apparently faster because it
doesn’t engage the Ruby code in `hub`.