You commited something and right after that you saw you forgot to remove log or add another file? Or you just want to extend your message? It is not necessary to revert the commit everytime.
How to add file to the last commit
You did some changes, added files and commited. Then you saw you forgot to add another file. What to do?
git add <the file>
git commit --amend --no-edit
--no-edit option just uses the original commit message without changing it or without launching an editor.
How to change commit message for the last commit
You commited something and you forgot to add an important note into the commit message? Or you want to add issue number? No problem...
git commit --amend
It will open your editor and let you change the message. Of course you can use
git commit --amend -m "commit message"
Do not use ammend if you have already pushed the commit to the repo and someone pulled it. This could, or better - probably will, cause problems.