Use custom gitmessage

- Custom git commit message template previously had to be symlinked to from the
  home directory, overriding the symlink created by thoughtbot's dotfiles.
- Submitted a PR to move the include statement in thoughtbot's .gitmessage to
  the end of the file.
- Now that we can customize the git message without linking to it from $HOME,
  we can move gitmessage to a git/ subdirectory of .dotfiles-local
Add aliases to gitconfig.