Use kdiff3 as your diff viewer in git.

Copy git-kdiff3 somewhere into your $PATH or your git exec path (usually /usr/libexec/git-core/):

sudo cp git-kdiff3 "`git --exec-path`"

Compare working copy against HEAD:

git kdiff3

Compare working copy against a specific commit/branch:

git kdiff3 $branch

Compare two commits:

git kdiff3 $commit1 $commit2