.gitconfig: rm "useless" core options

"Useless" because they are either the defaults, or internal settings, or something
you would rather set on a per-repository basis.
.gitconfig: steal settings from voku/dotfiles

Version: https://github.com/voku/dotfiles/blob/1734645/.gitconfig
.gitconfig: use 12 chars for short hash

This affects "git log" (and "git l"), the "git push" output, and pretty much every
place in Git where a short hash is used.
.gitconfig: add comment about color.ui