dgit: Drop an obsolete xxx comment

This comment comes originally from a WIP commit
  e0ebbc5879c01d4c3e802101e146c5fb08bc9d49
I have in before-rebase.2016-11-07.  In that commit, @merginputs
did not exist.  Now it does, and that is how the ref comes out.
So this comment is obsolete.

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