p4merge is Perforce Visual Merge and Diff Tools. download p4v.tgz for linux.

install p4merge

tar zxf p4v.tgz
sudo mv p4v-2012.1.500245/lib/p4v/ /lib/
sudo mv p4v-2012.1.500245/bin/p4merge* /bin/

config git

add following lines into ~/.gitconfig

[diff]
    tool = p4merge
[difftool "p4merge"]
    cmd = p4merge \\\"$LOCAL\\\" \\\"$REMOTE\\\"
[merge]
    tool = p4merge
[mergetool "p4merge"]
    cmd = p4merge \\\"$BASE\\\" \\\"$LOCAL\\\" \\\"$REMOTE\\\" \\\"$MERGED\\\"