git-gui: Remove combined diff showing behavior.

git-gui: Remove combined diff showing behavior.

The combined diff format can be very confusing, especially to new users
who may not even be familiar with a standard two way diff format.  So
for files which are already staged for commit and which are modifed in
the working directory we should show two different diffs, depending on
which side the user clicked on.

If the user clicks on the "Changes To Be Committed" side then we should
show them the PARENT<->index difference.  This is the set of changes they
will actually commit.

If the user clicks on the "Changed But Not Updated" side we should show
them the index<->working directory difference.  This is the set of changes
which will not be committed, as they have not been staged into the index.
This is especially useful when merging, as the "Changed But Not Updated"
files are the ones that need merge conflict resolution, and the diff here
is the conflict hunks and/or any evil merge created by the user.

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