git-gui: Make diff viewer colors match gitk's defaults.

git-gui: Make diff viewer colors match gitk's defaults.

Because users who use git-gui are likely to also be using gitk, we
should at least match gitk's default colors and formatting within the
diff viewer.

Unfortunately this meant that I needed to change the background colors
of the hunks in a 'diff --cc' output, as the green used for 'added line'
was completely unreadable on the old color.  We now use ivory1 to show
hunks which came from HEAD/parent^1, which are the portions that the
current branch has contributed, and are probably the user's own changes.
We use a very light blue for the portions which came from FETCH_HEAD,
as this makes the changes made by the other branch stand out more in the
diff.

I've also modified the hunk header lines to be blue, as that is how gitk
is showing them.

Apparently I forgot to raise the sel tag above everything else in the
diff viewer, which meant that selections in the diff viewer were not
visible if they were made on a 'diff --cc' hunk which had a background.
Its now the higest priority tag, ensuring the selection is always visible
and readable.

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