git-difftool originated in git-cola.

It first appeared in git.git's contrib/ area in January 2009
after it was rewritten as a GIT_EXTERNAL_DIFF compatible script.

git-difftool became an official git command in April 2009 with the
release of git 1.6.3.  setup.py checks for git 1.6.3 and installs
git-difftool when an older git version is detected.
