[gitg] Document why we call 'update_diff()' here

[Techlive Zheng <techlivezh src gnome org>]

commit 4c5a2f496bb5533f3d70a94bf8358cc1f6acf410
Author: Techlive Zheng <techlivezheng gmail com>
Date:   Tue Dec 10 18:09:32 2013 +0800

    Document why we call 'update_diff()' here
    
    https://bugzilla.gnome.org/show_bug.cgi?id=720726

 libgitg/gitg-diff-view.vala |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/libgitg/gitg-diff-view.vala b/libgitg/gitg-diff-view.vala
index 47a5818..1db62c2 100644
--- a/libgitg/gitg-diff-view.vala
+++ b/libgitg/gitg-diff-view.vala
@@ -291,6 +291,8 @@ namespace Gitg
                                return;
                        }
 
+                       // If both `d_diff` and `d_commit` are null, clear
+                       // the diff content
                        if (d_diff == null && d_commit == null)
                        {
                                run_javascript.begin("update_diff();", d_cancellable, (obj, res) => {