Problem

Forgot to add the project file to the previous commit.

git commit -m "Created a new context class to manage PolyPlane interaction"

Solution

Make your corrections and amend the HEAD

git add my_project_file.proj
git commit -C HEAD --amend

-C HEAD uses the commit message from the HEAD (the previous commit)

--amend includes the contents of this commit inside the previous commit

Use -c HEAD to open editor with previous commit message.