[git] how to git commit a whole folder?

When you “add” something in Git, you add it to the staging area. When you commit, you then commit what’s in the staging area, meaning it’s possible to commit only a sub-set of changed files at any one time.

In your case, you want to add the folder to the staging area, and then just do a normal commit:

$ git add foldername
$ git commit -m 'Helpful commit message'