Fix up 'dsf' git alias example in readme
Fix up 'dsf' git alias example in readme

- Fix quoting around GIT_PREFIX
- Use && to fail early if the cd does
- Use `git config --global alias.dsf`, which git recommends over
  hand-editing .gitconfig (and makes it easy to adapt the command to
  per-repository configs by dropping --global)

scottchiefbaker/diff-so-fancy,so-fancy/diff-so-fancy,volker-fr/diff-so-fancy,so-fancy/diff-so-fancy,scottchiefbaker/diff-so-fancy,volker-fr/diff-so-fancy