[gitg] Fix diff index against empty repository

[Jesse van den Kieboom <jessevdk src gnome org>]

commit f9ecad6db99faf02f416821922bc8991f0b285eb
Author: Jesse van den Kieboom <jessevdk gnome org>
Date:   Wed Aug 17 16:02:30 2016 +0200

    Fix diff index against empty repository
    
    https://bugzilla.gnome.org/show_bug.cgi?id=769996

 gitg/commit/gitg-commit.vala |   15 ++++++++++-----
 libgitg/gitg-stage.vala      |    7 ++++++-
 2 files changed, 16 insertions(+), 6 deletions(-)
---
diff --git a/gitg/commit/gitg-commit.vala b/gitg/commit/gitg-commit.vala
index 5412adf..7b6d34a 100644
--- a/gitg/commit/gitg-commit.vala
+++ b/gitg/commit/gitg-commit.vala
@@ -1149,13 +1149,18 @@ namespace GitgCommit
                        opts.n_context_lines = 3;
                        opts.n_interhunk_lines = 3;
 
-                       var stage = application.repository.stage;
+                       var repository = application.repository;
+
+                       var stage = repository.stage;
 
-                       Ggit.Tree tree;
+                       Ggit.Tree? tree = null;
 
                        try
                        {
-                               tree = yield stage.get_head_tree();
+                               if (!repository.is_empty())
+                               {
+                                       tree = yield stage.get_head_tree();
+                               }
                        }
                        catch { return null; }
 
@@ -1163,10 +1168,10 @@ namespace GitgCommit
 
                        try
                        {
-                               var index = application.repository.get_index();
+                               var index = repository.get_index();
 
                                yield Gitg.Async.thread(() => {
-                                       diff = new Ggit.Diff.tree_to_index(application.repository,
+                                       diff = new Ggit.Diff.tree_to_index(repository,
                                                                           tree,
                                                                           index,
                                                                           opts);
diff --git a/libgitg/gitg-stage.vala b/libgitg/gitg-stage.vala
index cc4f63c..95f8123 100644
--- a/libgitg/gitg-stage.vala
+++ b/libgitg/gitg-stage.vala
@@ -1009,7 +1009,12 @@ public class Stage : Object
                        opts.new_prefix = defopts.new_prefix;
                }
 
-               var tree = yield get_head_tree();
+               Ggit.Tree? tree = null;
+
+               if (!d_repository.is_empty())
+               {
+                       tree = yield get_head_tree();
+               }
 
                return new Ggit.Diff.tree_to_index(d_repository,
                                                   tree,