Setup p4merge for sorcetree external diff
#
# diff --git a/git/.gitconfig b/git/.gitconfig
# index 3abcd3e..9cbb725 100644
# --- a/git/.gitconfig
# +++ b/git/.gitconfig
# @@ -42,3 +42,9 @@
#    difff = diff --word-diff
#  [init]
#  	templatedir = ~/.git_template
# +[difftool "sourcetree"]
# +	cmd = /Applications/p4merge.app/Contents/MacOS/p4merge \"$LOCAL\" \"$REMOTE\"
# +	path =
# +[mergetool "sourcetree"]
# +	cmd = /Applications/p4merge.app/Contents/MacOS/p4merge \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"
# +	trustExitCode = true