git-gui: Allow adding untracked files in selection.

git-gui: Allow adding untracked files in selection.

The previous implementation of do_include_selection did not actually
add files in state _O (untracked, not added) into the repository when
they were in the selection and Commit->Include Selected Files was used.
This was due to the file state filtering logic being the same as that
of Commit->Include All Files, which only considers existing files.

Also fixed a minor issue with rejected attempts to amend an initial
commit.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>