git: Remove misplaced .gitignore

~/.config/git/.gitignore should have never been committed.
git: alias graph = log --graph --decorate --oneline
git: alias root = rev-parse --show-toplevel
git: Remove non-existent option ui.color
git: Disable rerere

I don't want this enabled globally.
git: Change merge.tool from gvimdiff -> vimdiff

I often work through ssh, where gvimdiff can't work.
git: Ignore build-aux, an autotools crumb