test: enforce master to be the default branch in test

Newer git issue a message about this.

Differential Revision: https://phab.mercurial-scm.org/D10281

tests/test-git-interop.t

@@ -14,6 +14,7 b' Setup:'
   >    git commit "$@" >/dev/null 2>/dev/null || echo "git commit error"
   >    count=`expr $count + 1`
   >  }
+  $ git config --global init.defaultBranch master
 
 
   $ hg version -v --config extensions.git= | grep '^[E ]'