git-gui: Add menu option to include only selected files.

git-gui: Add menu option to include only selected files.

When the user selects a number of files they would typically expect
to be able to act on that selection, such as by including those files
into the next commit.

So we now have a menu option under the Commit menu that lets the user
include only the selection, rather than everything.  If there is no
selection but there is a file in the diff viewer than we consider that
to be the selection (a selection of 1).  Unfortunately we don't disable
this option yet when there's nothing selected to include, but this is
probably not a big deal as there are very few situations where there
are no selected files.

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