do not propose to add to or remove from git files that do not need to.

When adding an existing file to a project, QtCreator proposes to add it to git (when the current project is managed with git, of course). If the file is already known to git and it is not modified, this proposition should not be done.

When removing a file from a project, QtCreator proposes to remove it from git. If the file is not in the git repository in the first place, this proposition should not be done.