When committing, you should always check the diff of your changes so you don't include any leftovers or irrelevant/bad changes. Any time spent on that is well invested as it saves you rejected merge requests or any mess you need to clean up later.

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While you can just use git diff or git diff --cached (to diff staged changes), you can also have Git include the changes as a comment below the commit message:

git commit -v

That will open up the usual commit "file" in your preferred text editor, but it will include a diff below the status comment. Vim will understand and properly highlight the diff.

You should go ahead and define an alias (in your ~/.bashrc) that you can use to quickly commit and always include the diff. These are mine:

alias gc='git commit -v'
alias gca='git commit -v -a'

Note that the -v switch will also work when you --amend to a commit. It will always show the full diff of that commit, which is very nice.