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/git/gitconstants.h

@@ -53,5 +53,7 @@ const char C_GITEDITORID[]  = "Git Editor";
 
 const int OBSOLETE_COMMIT_AGE_IN_DAYS = 90;
 
+const char EXPAND_BRANCHES[] = "Branches: <Expand>";
+
 } // namespace Constants
 } // namespace Git