Move git specific stuff out of diff editor plugin

Move it to the git plugin.

Change-Id: I8151573ed50df70776f7ebf0475dd41fb84fae83
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/diffeditor/diffeditorconstants.h

@@ -41,7 +41,5 @@ const char UNIFIED_VIEW_ID[] = "DiffEditor.Unified";
 
 const char G_TOOLS_DIFF[] = "QtCreator.Group.Tools.Options";
 
-const char EXPAND_BRANCHES[] = "Branches: <Expand>";
-
 } // namespace Constants
 } // namespace DiffEditor