How do i see the differences between a previous git commit and what you are about to commit?

shoebill

also see:
Man page git-diff.

-p, --patch
	   Interactively choose hunks of patch between the index and the work tree and add them
	   to the index. This gives the user a chance to review the difference before adding
	   modified contents to the index.

	   This effectively runs add --interactive, but bypasses the initial command menu and
	   directly jumps to the patch subcommand. See “Interactive mode” for details.

The command will run through each of the changes Prompting for action to be applied.

$ git add -p