Switch mergetool to git alias

There's a chance for name collisions between aliases and actual commands
(gs is causing one for sure). Switch merge tool shell alias to a git
alias accessible through g mt.
Switch alias to commands I use

I wasn't really using the aliases as they are, I even wrote gs to show
me status -uno, when all I really needed to do was change the alias
below.
Remove shell alias gs

No longer required as git st performs the same function without the
collision with ghostscript.