Add git-credit by Zach Holman

See https://github.com/holman/dotfiles/blob/master/bin/git-credit
Add git-rank-contributers

From https://github.com/holman/dotfiles/blob/master/bin/git-rank-contributers
Add git-amend

Stolen from Zach
Add git-promote

Promotes a local topic branch to a remote tracking branch of the same name,
by pushing and then setting up the git config. Stolen from Zach Holman.
Add git-wtf