git-gui: By default don't allow partially included files.

git-gui: By default don't allow partially included files.

The concept of the Git index is confusing for many users, especially
those who are newer to Git.

Since git-gui is (at least partially) intended to be used by newer
users who don't need the complexity of the index to be put in front
of them early on, we should hide it by making any partially included
file fully included as soon as we identify it.  To do this we just
run a quick update_index pass on any file which differs both in the
index and the working directory, as these files have already been
at least partially included by the user.

A new option has been added in the options dialog (gui.partialinclude)
which lets the user enable accessing the index from git-gui.  This
just disables the automatic update_index pass on partially included
files.

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