[gnome-builder] build: bump libgit2 in nightly

[Christian Hergert <chergert src gnome org>]

commit bfc44596634ccac0c747a586fc53cc7d6dd64ddf
Author: Christian Hergert <chergert redhat com>
Date:   Fri Nov 29 09:17:00 2019 -0800

    build: bump libgit2 in nightly

 build-aux/flatpak/org.gnome.Builder.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 131058918..87b02e25b 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -229,7 +229,7 @@
                 {
                     "type" : "git",
                     "url" : "https://github.com/libgit2/libgit2.git";,
-                    "branch" : "maint/v0.27"
+                    "branch" : "maint/v0.28"
                 }
             ]
         },