worktree support: dgit: Fix for .git/logs/refs/

Experimentally, both the common dir and the worktree's git dir have a
.git/logs.  But, as might be expected, the worktree's one has the
reflog for its HEAD and not any of the refs/ reflogs.

Implicitly, we are deciding here that the dgit quilt cache is shared
between all worktrees.  That seems fine.  I think we don't ever rely
on its actual current value, so even concurrent dgit runs ought to be
fine.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>