Add `afk` alias in Bash to lock screen.

Idea from https://github.com/mathiasbynens/dotfiles/blob/master/.aliases
Add some git aliases from Gary Bernhardt.

From https://github.com/garybernhardt/dotfiles/blob/master/.gitconfig
Didn't include the ones requiring external git utilities.
Use Shift+Up for `history-search-backward` instead of Up.

Leave unshifted Up keybinding alone, so others are not confused.
Control or Option cursor keys move by word in Readline.