Add option to remove file from commit in Amend Last Commit

When a whole file needs to be removed from a commit, the usual way is to reset and commit again. This has several drawbacks, e.g. the commit message must be stored and copied back, dates change etc.

git-gui has the possibility to amend and remove files from the list. A similar approach would be cool for Creator.

Maybe it would also be good to allow unstaging chunks or lines.