Occasionally I want to diff one or more files in git and, if everything looks good, then add them. To do so, I often take advantage of bash's ^ notation to do a replacement:

user$ git diff -- file1.txt file2.txt
user$ ^diff^add
user$ git commit -m "Looks good to me"

The second command replaces diff with add turning the command into git add -- file1.txt file2.txt without having to retype the entire command.