FileMerge (opendiff) can really come in handy when you need to visually compare merging conflicts. Other times it's just a nice visual way to review your days work.
The following method works by creating a simple bash script (git-diff-cmd.sh) that sets us up with the proper command line arguments for Git to pass off files to FileMerge.
Check that your shell can run opendiff. You may need to:
sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer/
Create the bash script:
vim ~/dev/bin/git-diff-cmd.sh
Paste the following and save it:
#!/bin/sh
opendiff "$2" "$5" -merge "$1"
Make the bash script executable:
chmod +x ~/dev/bin/git-diff-cmd.sh
Tell Git (globally) to run our bash script when 'git diff' is issued:
git config --global diff.external ~/dev/bin/git-diff-cmd.sh
Now head over to your Git-aware project directory and issue a
git diff /path/to/modified/file.rb and FileMerge will pop up showing you the
differences against it and HEAD.
You can also do things like git diff --cached to review all the changes
against HEAD.