git-gui: fallback right pane to packed widgets with Tk 8.4

Since 918dbf5, git-gui crashes if started with Tk 8.4. The reason is that
tk < 8.5 does not support -stretch option for panedwindow.

Without the option it's not possible to properly expand the right half -
the commit area is expanded, while desired behavior is to expand the diff
area. So the whole feature should be disabled with Tk
version less than 8.5.

Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -3224,18 +3224,28 @@ unset i
 
 # -- Diff and Commit Area
 #
-${NS}::panedwindow .vpane.lower -orient vertical
-${NS}::frame .vpane.lower.commarea
-${NS}::frame .vpane.lower.diff -relief sunken -borderwidth 1 -height 500
-.vpane.lower add .vpane.lower.diff
-.vpane.lower add .vpane.lower.commarea
-.vpane add .vpane.lower
-if {$use_ttk} {
-	.vpane.lower pane .vpane.lower.diff -weight 1
-	.vpane.lower pane .vpane.lower.commarea -weight 0
+if {$have_tk85} {
+	${NS}::panedwindow .vpane.lower -orient vertical
+	${NS}::frame .vpane.lower.commarea
+	${NS}::frame .vpane.lower.diff -relief sunken -borderwidth 1 -height 500
+	.vpane.lower add .vpane.lower.diff
+	.vpane.lower add .vpane.lower.commarea
+	.vpane add .vpane.lower
+	if {$use_ttk} {
+		.vpane.lower pane .vpane.lower.diff -weight 1
+		.vpane.lower pane .vpane.lower.commarea -weight 0
+	} else {
+		.vpane.lower paneconfigure .vpane.lower.diff -stretch always
+		.vpane.lower paneconfigure .vpane.lower.commarea -stretch never
+	}
 } else {
-	.vpane.lower paneconfigure .vpane.lower.diff -stretch always
-	.vpane.lower paneconfigure .vpane.lower.commarea -stretch never
+	frame .vpane.lower -height 300 -width 400
+	frame .vpane.lower.commarea
+	frame .vpane.lower.diff -relief sunken -borderwidth 1
+	pack .vpane.lower.diff -fill both -expand 1
+	pack .vpane.lower.commarea -side bottom -fill x
+	.vpane add .vpane.lower
+	.vpane paneconfigure .vpane.lower -sticky nsew
 }
 
 # -- Commit Area Buttons