Support more fish abbr's for git

I'm sure that there is a fish package that supports git aliases or
abbreviations. This time, I'm preferring to write my own.

Side note: The more that I use fish's abbreviations, the less that I like them
-- at least for the way in which I'm using them.

configs/shared/.config/fish/config.fish

@@ -179,6 +179,8 @@ abbr --add g     hub
 abbr --add git   hub
 abbr --add ga    git add
 abbr --add gc    git commit
+abbr --add gco   git checkout
+abbr --add gd    git diff
 abbr --add gp    git push
 abbr --add grbi  git rebase --interactive
 abbr --add grba  git rebase --abort