If you are using git 2.9, this can be done with a config.
git config --global commit.verbose true
Git 2.9.0 Release Notes:
https://github.com/git/git/blob/v2.9.0/Documentation/RelNotes/2.9.0.txt
“git commit” learned to pay attention to the “commit.verbose”
configuration variable and act as if the “–verbose” option
was given from the command line.