Make updating diff asynchronously and be called on Idle

Before this commit, huge changes will affect the performance of the application
A lot due to fetching the changes and showing them on the screen.

This commit helps in reducing the lag a bit, by moving the updating and buffering
The changes to Idle.

libgitg/gitg-diff-view.vala

@@ -57,6 +57,8 @@ public class Gitg.DiffView : Gtk.Grid
 	private uint d_reveal_options_timeout;
 	private uint d_unreveal_options_timeout;
 
+	private uint d_update_diff_id;
+
 	private static Gee.HashSet<string> s_image_mime_types;
 
 	public Ggit.DiffOptions options
@@ -231,6 +233,12 @@ public class Gitg.DiffView : Gtk.Grid
 			d_cancellable.cancel();
 		}
 
+		if(d_update_diff_id != 0)
+		{
+			Source.remove(d_update_diff_id);
+			d_update_diff_id = 0;
+		}
+
 		base.dispose();
 	}
 
@@ -590,7 +598,22 @@ public class Gitg.DiffView : Gtk.Grid
 
 		if (d_diff != null)
 		{
-			update_diff(d_diff, preserve_expanded, d_cancellable);
+			if(d_update_diff_id != 0)
+			{
+				Source.remove(d_update_diff_id);
+				d_update_diff_id = 0;
+			}
+
+			d_update_diff_id = Idle.add(() => {
+				update_diff.begin(d_diff,
+								  preserve_expanded,
+								  d_cancellable,
+								  (obj, res) => {
+								  	d_update_diff_id = 0;
+								  });
+
+				return false;
+			});
 		}
 	}
 
@@ -692,7 +715,7 @@ public class Gitg.DiffView : Gtk.Grid
 		return "";
 	}
 
-	private void update_diff(Ggit.Diff diff, bool preserve_expanded, Cancellable? cancellable)
+	private async void update_diff(Ggit.Diff diff, bool preserve_expanded, Cancellable? cancellable)
 	{
 		var nqueries = 0;
 		var finished = false;