If you want to commit all changes - both new files and modified files then use the following command:

git add --all && git commit -m "commit message"

Alternatively this command achieves the same result:

git add . && git commit -m "commit message"

Even though the following command looks to achieves the same result, but it does not. Following command only adds the modified files and ignores newly added files:

git -am "commit message"