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>

git-gui.sh

@@ -3916,24 +3916,24 @@ pack $ui_diff -side left -fill both -expand 1
 pack .vpane.lower.diff.header -side top -fill x
 pack .vpane.lower.diff.body -side bottom -fill both -expand 1
 
-$ui_diff tag conf d_@ -font font_diffbold
-$ui_diff tag conf d_+ -foreground blue
+$ui_diff tag conf d_@ -foreground blue -font font_diffbold
+$ui_diff tag conf d_+ -foreground {#00a000}
 $ui_diff tag conf d_- -foreground red
 
-$ui_diff tag conf d_++ -foreground blue
+$ui_diff tag conf d_++ -foreground {#00a000}
 $ui_diff tag conf d_-- -foreground red
 $ui_diff tag conf d_+s \
-	-foreground blue \
-	-background azure2
+	-foreground {#00a000} \
+	-background {#e2effa}
 $ui_diff tag conf d_-s \
 	-foreground red \
-	-background azure2
+	-background {#e2effa}
 $ui_diff tag conf d_s+ \
-	-foreground blue \
-	-background {light goldenrod yellow}
+	-foreground {#00a000} \
+	-background ivory1
 $ui_diff tag conf d_s- \
 	-foreground red \
-	-background {light goldenrod yellow}
+	-background ivory1
 
 $ui_diff tag conf d<<<<<<< \
 	-foreground orange \
@@ -3945,6 +3945,8 @@ $ui_diff tag conf d>>>>>>> \
 	-foreground orange \
 	-font font_diffbold
 
+$ui_diff tag raise sel
+
 # -- Diff Body Context Menu
 #
 set ctxm .vpane.lower.diff.body.ctxm