git basics demo: git add + git commit instead of git commit <filename>

[mwhosek]

In the current lesson I use "git commit -m " to commit changes to a file, but the formal best practice is to use the two-step "git add ", "git commit -m ".

git commit is a shortcut that git doesn't allow to resolve merge conflicts. So, it is confusing when we do the merge conflict that we switch back to git add + git commit at the end of the lesson, after we are doing the shortcut everywhere else.